/* ════════════════════════════════════════════════════════════
     SoloCogs · Atomic Vault (Phase 2)
     A periodic-table vault. Locked cells render with a desaturated
     "Cryo" effect; unlocked ones glow with their group colour.
     Reads from SoloProgression.unlockedSymbols. Click any unlocked
     element to open the Researcher's Log modal.
     ════════════════════════════════════════════════════════════ */
  /* ── Midnight Vault - dark-mode environment ──
     The Vault is intentionally a different visual world from the rest of
     the platform: a Deep Midnight charcoal page that lets dormant tiles
     fade back and discovered ones glow with neon group colour. */
  body {
    background:
      radial-gradient(ellipse 1200px 600px at 50% -10%, rgba(45, 212, 191, 0.06) 0%, transparent 60%),
      #0F172A;
    color: #E2E8F0;
  }

  /* ── Page header ── */
  .arc-hero {
    position: relative;
    background: rgba(15, 23, 42, 0.65);
    padding: 3.5rem 1.5rem 3rem;
    border-bottom: 1px solid rgba(148, 163, 184, 0.12);
  }
  .arc-hero-inner { max-width: 1280px; margin: 0 auto; }
  /* Reset the global `nav { background: var(--white); ... }` rule from
     styles.css so our breadcrumb doesn't render as a sticky white bar
     across the dark Atomic Vault hero. */
  .arc-breadcrumb {
    display: inline-flex;
    align-items: center;
    flex-wrap: wrap;
    gap: 0.4rem;
    font-family: var(--solo-font-head);
    font-size: 0.82rem;
    font-weight: 600;
    margin-bottom: 1.4rem;
    background: transparent;
    border: 0;
    padding: 0;
    position: static;
    height: auto;
    z-index: auto;
  }
  .arc-breadcrumb a {
    color: #5EEAD4;
    text-decoration: none;
  }
  .arc-breadcrumb a:hover { text-decoration: underline; text-decoration-thickness: 2px; text-underline-offset: 4px; }
  .arc-breadcrumb-sep { color: rgba(255, 255, 255, 0.85); }
  .arc-breadcrumb-current { color: #FFFFFF; }
  .arc-eyebrow {
    display: inline-flex; align-items: center; gap: 0.45rem;
    font-family: var(--solo-font-head); font-size: 0.7rem; font-weight: 700;
    color: rgba(148, 163, 184, 0.85); letter-spacing: 0.12em; text-transform: uppercase;
    margin-bottom: 0.75rem;
  }
  .arc-eyebrow::before {
    content: ''; width: 8px; height: 8px; border-radius: 50%;
    background: #2DD4BF; box-shadow: 0 0 0 3px rgba(45,212,191,.25), 0 0 12px rgba(45,212,191,.6);
  }
  .arc-hero h1 {
    font-family: var(--solo-font-head); font-weight: 700;
    font-size: clamp(2.4rem, 5vw, 3.4rem); line-height: 1.05;
    letter-spacing: -0.02em; margin: 0 0 0.75rem;
    color: #F8FAFC;
  }
  .arc-hero-lede {
    font-family: var(--solo-font-body);
    font-size: clamp(0.98rem, 1.6vw, 1.08rem);
    line-height: 1.6; color: #CBD5E1; margin: 0;
    max-width: 60ch;
  }

  /* ── Stats strip ── */
  .arc-stats {
    display: flex; gap: 1.25rem; flex-wrap: wrap;
    margin: 1.6rem 0 0; padding: 1rem 1.2rem;
    background: rgba(30, 41, 59, 0.6);
    border: 1px solid rgba(148, 163, 184, 0.15);
    border-radius: 14px;
    align-items: center;
    backdrop-filter: blur(8px);
  }
  .arc-puzzle {
    margin: 1.6rem 0 .25rem;
    background: linear-gradient(135deg, rgba(99, 102, 241, 0.18) 0%, rgba(45, 212, 191, 0.18) 100%);
    border: 1px solid rgba(148, 163, 184, 0.25);
    border-radius: 14px;
    padding: 1rem 1.2rem 1.2rem;
    color: #F1F5F9;
  }
  .arc-puzzle__head { margin-bottom: .65rem; }
  .arc-puzzle__eyebrow {
    font-family: 'Lexend', sans-serif;
    font-size: .72rem; font-weight: 700;
    letter-spacing: .12em; text-transform: uppercase;
    color: #C7D2FE;
  }
  .arc-puzzle__title {
    font-family: 'Lexend', sans-serif;
    font-weight: 600; font-size: 1rem;
    color: #F8FAFC; margin: .3rem 0 0;
  }
  .arc-puzzle__body { font-size: .92rem; line-height: 1.55; color: #E2E8F0; }
  .arc-puzzle__row { display: flex; flex-wrap: wrap; gap: .65rem; margin-top: .85rem; }
  .arc-puzzle__opt {
    flex: 1; min-width: 90px;
    background: rgba(15, 23, 42, 0.5);
    color: #F1F5F9;
    border: 1.5px solid rgba(148, 163, 184, 0.32);
    padding: .65rem .85rem;
    border-radius: 12px;
    font-family: 'Lexend', sans-serif;
    font-weight: 700; font-size: .95rem;
    cursor: pointer; min-height: 44px;
    transition: transform .12s ease, background .12s ease;
  }
  .arc-puzzle__opt:hover { background: rgba(15, 23, 42, 0.7); transform: translateY(-1px); }
  .arc-puzzle__opt.is-correct {
    background: #166534; border-color: #4ADE80; color: #ECFCCB;
  }
  .arc-puzzle__opt.is-wrong {
    background: #7F1D1D; border-color: #FCA5A5; color: #FEE2E2;
  }
  .arc-puzzle__feedback {
    margin-top: .85rem; font-size: .9rem; color: #FBBF24;
    min-height: 1.2em;
  }
  .arc-puzzle__feedback.is-correct { color: #BBF7D0; }
  /* XP chip rendered inline in the correct-answer feedback so the student
     can see the reward they earned alongside the research-note flavour. */
  .arc-puzzle__xp {
    display: inline-block;
    margin: 0 .15rem;
    padding: .15rem .55rem;
    background: rgba(45, 212, 191, .18);
    border: 1px solid rgba(45, 212, 191, .55);
    border-radius: 999px;
    color: #2DD4BF;
    font-family: var(--solo-font-head, 'Lexend', sans-serif);
    font-weight: 700;
    font-size: .78rem;
    letter-spacing: .02em;
    vertical-align: 1px;
  }
  .arc-purpose {
    margin: .85rem 0 0;
    background: rgba(30, 41, 59, 0.55);
    border: 1px solid rgba(148, 163, 184, 0.15);
    border-radius: 14px;
    backdrop-filter: blur(8px);
    color: #E2E8F0;
    overflow: hidden;
  }
  .arc-purpose > summary {
    list-style: none;
    cursor: pointer;
    padding: .85rem 1.2rem;
    display: flex; align-items: center; gap: .75rem;
    font-family: var(--solo-font-head, 'Lexend', sans-serif);
    font-weight: 600; font-size: .95rem;
    color: #F1F5F9;
    user-select: none;
  }
  .arc-purpose > summary::-webkit-details-marker { display: none; }
  .arc-purpose__icon {
    width: 28px; height: 28px; border-radius: 999px;
    background: linear-gradient(135deg, #2DD4BF 0%, #14B8A6 100%);
    color: #0F172A;
    display: inline-flex; align-items: center; justify-content: center;
    font-weight: 800; font-size: 1.05rem;
    flex-shrink: 0;
  }
  .arc-purpose__label { flex: 1; }
  .arc-purpose__chevron {
    transition: transform .18s ease;
    color: #94A3B8;
  }
  .arc-purpose[open] .arc-purpose__chevron { transform: rotate(180deg); }
  .arc-purpose__body {
    padding: .25rem 1.2rem 1.1rem;
    border-top: 1px dashed rgba(148, 163, 184, 0.18);
  }
  .arc-purpose__body p {
    margin: .85rem 0 0;
    font-size: .92rem; line-height: 1.55;
    color: #CBD5E1;
  }
  .arc-purpose__body strong { color: #F1F5F9; }
  .arc-purpose__note {
    font-size: .82rem !important; font-style: italic;
    color: #94A3B8 !important;
  }
  .arc-stat { display: flex; flex-direction: column; gap: 0.15rem; }
  .arc-stat-label { font-family: var(--solo-font-head); font-size: 0.62rem; font-weight: 700; color: rgba(148, 163, 184, 0.85); letter-spacing: 0.12em; text-transform: uppercase; }
  .arc-stat-value { font-family: var(--solo-font-head); font-size: 1.25rem; font-weight: 700; color: #F8FAFC; letter-spacing: -0.01em; }
  .arc-stat-value strong { color: #2DD4BF; text-shadow: 0 0 12px rgba(45, 212, 191, 0.6); }
  .arc-stat-divider { width: 1px; height: 36px; background: rgba(148, 163, 184, 0.2); flex-shrink: 0; }

  /* ── Periodic Vault grid ── */
  .arc-vault-section { padding: 2.5rem 1.5rem 4rem; }
  .arc-vault-shell { max-width: 1280px; margin: 0 auto; }
  .arc-grid {
    display: grid;
    grid-template-columns: repeat(18, minmax(0, 1fr));
    grid-template-rows: repeat(7, minmax(54px, auto)) 0.65rem repeat(2, minmax(54px, auto));
    gap: 6px;
    overflow-x: auto;
    padding-bottom: 6px;
  }
  .arc-grid-spacer { grid-row: 8; grid-column: 1 / -1; }

  /* ── Central Discovery Progress bar - sits in the empty row 1–2,
        cols 3–12 dead-space between H and the p-block. Top-aligned so
        it occupies row 1 only; row 2 below it stays empty as natural
        breathing room before the alkali/alkaline-earth row 3. */
  .arc-progress {
    grid-row: 1 / span 2;
    grid-column: 5 / span 6;
    align-self: start;
    justify-self: center;
    width: 100%;
    max-width: 480px;
    display: flex;
    flex-direction: column;
    justify-content: center;
    gap: 0.3rem;
    padding: 0.45rem 0.85rem;
    background: linear-gradient(180deg, rgba(30, 41, 59, 0.7) 0%, rgba(15, 23, 42, 0.55) 100%);
    border: 1px solid rgba(45, 212, 191, 0.20);
    border-radius: 10px;
    box-shadow:
      0 0 0 1px rgba(45, 212, 191, 0.10) inset,
      0 14px 32px -16px rgba(45, 212, 191, 0.40);
    min-width: 0;
  }
  .arc-progress-head {
    display: flex; align-items: baseline; justify-content: space-between; gap: 0.75rem;
    min-width: 0;
  }
  .arc-progress-label {
    font-family: var(--solo-font-head);
    font-size: 0.58rem; font-weight: 700;
    color: rgba(148, 163, 184, 0.85);
    letter-spacing: 0.14em; text-transform: uppercase;
    white-space: nowrap;
  }
  .arc-progress-value {
    font-family: var(--solo-font-head);
    font-size: 0.72rem; font-weight: 700;
    color: #F8FAFC;
    letter-spacing: 0.02em;
    white-space: nowrap;
  }
  .arc-progress-value strong {
    color: #2DD4BF;
    text-shadow: 0 0 10px rgba(45, 212, 191, 0.55);
  }
  .arc-progress-track {
    position: relative;
    height: 7px;
    background: rgba(15, 23, 42, 0.75);
    border: 1px solid rgba(148, 163, 184, 0.18);
    border-radius: 999px;
    overflow: hidden;
  }
  /* ── Energy Conduit ──────────────────────────────────────
     The fill is a flowing gradient that animates left-to-right
     so the bar always reads as "energy is moving". A second
     forward-tilted highlight stripe slides across at a faster
     rate to add a visible current. */
  .arc-progress-fill {
    position: absolute; inset: 0 auto 0 0;
    width: 0%;
    background:
      linear-gradient(110deg,
        rgba(255, 255, 255, 0.0)  0%,
        rgba(255, 255, 255, 0.18) 38%,
        rgba(255, 255, 255, 0.55) 50%,
        rgba(255, 255, 255, 0.18) 62%,
        rgba(255, 255, 255, 0.0)  100%) 0 0 / 80px 100% repeat-x,
      linear-gradient(90deg,
        #14B8A6 0%, #2DD4BF 25%, #5EEAD4 50%, #2DD4BF 75%, #14B8A6 100%) 0 0 / 200% 100%;
    border-radius: 999px;
    box-shadow: 0 0 12px rgba(45, 212, 191, 0.65);
    transition: width 0.6s cubic-bezier(0.22, 1, 0.36, 1);
    animation:
      arc-conduit-flow      4s linear   infinite,
      arc-conduit-highlight 2.4s linear infinite;
  }
  @keyframes arc-conduit-flow {
    0%   { background-position: 0 0, 0 0; }
    100% { background-position: 0 0, 200% 0; }
  }
  @keyframes arc-conduit-highlight {
    0%   { background-position: -120px 0, 0 0; }
    100% { background-position:  120px 0, 0 0; }
  }
  /* prefers-reduced-motion: kill every infinite animation on the Vault,
     not just the progress-fill. Previously: arc-mystery-glitch,
     arc-next-pulse, arc-group-pulse, arc-reaction-burst kept firing,
     which fails the SEND-first claim for vestibular-sensitive users. */
  @media (prefers-reduced-motion: reduce) {
    .arc-progress-fill,
    .arc-progress-next.is-mystery,
    .arc-tile:hover,
    .arc-tile.is-pulsing,
    [class*="arc-"][class*="-pulse"],
    [class*="arc-"][class*="-glitch"],
    [class*="arc-"][class*="-burst"] {
      animation: none !important;
      transform: none !important;
    }
  }
  .arc-progress-foot {
    display: flex; align-items: center; justify-content: space-between; gap: 0.5rem;
    font-family: var(--solo-font-head);
    font-size: 0.7rem; font-weight: 700;
    color: rgba(226, 232, 240, 0.85);
    letter-spacing: 0.04em;
    min-width: 0;
  }
  .arc-progress-foot .arc-progress-current {
    color: #2DD4BF;
    text-shadow: 0 0 10px rgba(45, 212, 191, 0.55);
  }
  .arc-progress-foot .arc-progress-next {
    color: rgba(148, 163, 184, 0.85);
  }
  .arc-progress-foot .arc-progress-arrow {
    flex: 1; text-align: center;
    color: rgba(148, 163, 184, 0.55);
  }
  /* ── Next-action CTA ──────────────────────────────────────
     Below the foot row. Tells the student EXACTLY how much XP
     they need + gives them a one-click route to earn it.
     Persona-driven: Maya (high achiever, wants the target),
     Sam (structure-loving completionist, wants "what now?"). */
  .arc-progress-cta {
    display: flex; align-items: center; justify-content: space-between;
    flex-wrap: wrap;
    gap: 0.4rem 0.6rem;
    margin-top: 0.5rem;
    font-family: var(--solo-font-head);
    /* Bumped from 0.68rem (~10.9px) - failed WCAG 1.4.4 body text floor
       and was hostile to dyslexic readers. 0.85rem (~13.6px) is the
       smallest acceptable in-Vault size + still respects the compact
       Discovery Progress strip layout. */
    font-size: 0.85rem; font-weight: 600;
    letter-spacing: 0.02em;
  }
  .arc-progress-cta-text {
    color: rgba(226, 232, 240, 0.78);
  }
  .arc-progress-cta-text strong {
    color: #2DD4BF;
    text-shadow: 0 0 10px rgba(45, 212, 191, 0.55);
  }
  .arc-progress-cta-link {
    display: inline-flex; align-items: center; gap: 0.25rem;
    padding: 0.25rem 0.65rem;
    border-radius: 999px;
    background: rgba(45, 212, 191, 0.14);
    border: 1px solid rgba(94, 234, 212, 0.45);
    color: #5EEAD4;
    text-decoration: none;
    transition: background 0.18s ease, border-color 0.18s ease, transform 0.18s ease;
  }
  .arc-progress-cta-link:hover,
  .arc-progress-cta-link:focus-visible {
    background: rgba(45, 212, 191, 0.22);
    border-color: rgba(94, 234, 212, 0.75);
    transform: translateY(-1px);
    outline: none;
  }
  .arc-progress-cta[hidden] { display: none; }
  /* Mystery glitch glyph - used in place of the next-element symbol when the
     student hasn't unlocked it yet. A monospaced "?" that flickers between
     teal/slate and shimmies once per beat to read as scrambled / unknown.

     Stabilization: as the student approaches 100% XP, the renderer writes
     --stab (0..1) on the element. At 0 the glitch is full intensity in
     teal; near 1 the jitter dampens out and the colour shifts toward the
     NEXT element's group colour (--next-color). The element starts to
     "lock in" before it actually crystallises at level-up. */
  .arc-progress-next.is-mystery {
    --stab: 0;
    --next-color: #5EEAD4;
    font-family: 'JetBrains Mono', monospace;
    color: color-mix(in srgb, rgba(94, 234, 212, 0.95) calc((1 - var(--stab)) * 100%), var(--next-color) calc(var(--stab) * 100%));
    text-shadow:
      0 0 calc(8px + 6px * var(--stab))
      color-mix(in srgb, rgba(45, 212, 191, 0.55) calc((1 - var(--stab)) * 100%), var(--next-color) calc(var(--stab) * 100%));
    animation: arc-mystery-glitch 2.4s steps(1, end) infinite;
    /* Damp the jitter as stabilisation rises - 0% jitter at full --stab. */
    animation-duration: calc(2.4s + 4s * var(--stab));
    animation-timing-function: cubic-bezier(.4, 0, .2, 1);
    display: inline-block;
    min-width: 1ch;
  }
  .arc-progress-next.is-mystery.is-stabilised {
    animation: none;
    transform: none !important;
  }
  @keyframes arc-mystery-glitch {
    0%, 100% { transform: translate(0, 0);
               color: rgba(94, 234, 212, 0.95);
               text-shadow: 0 0 8px rgba(45, 212, 191, 0.55); }
    35%      { transform: translate(0.8px, -0.6px);
               color: rgba(148, 163, 184, 0.85);
               text-shadow: 0 0 4px rgba(148, 163, 184, 0.45),
                            -1px 0 rgba(94, 234, 212, 0.7); }
    52%      { transform: translate(-1px, 0.5px);
               color: rgba(94, 234, 212, 0.9);
               text-shadow: 1px 0 rgba(252, 165, 165, 0.4),
                            -1px 0 rgba(94, 234, 212, 0.6); }
    68%      { transform: translate(0, 0);
               color: rgba(94, 234, 212, 0.95); }
  }
  @media (max-width: 760px) {
    .arc-progress {
      grid-column: 1 / -1;
      grid-row: auto;
      padding: 0.55rem 0.75rem;
    }
  }

  /* "See lanthanides/actinides" placeholder cells in rows 6 and 7, col 3 */
  .arc-placeholder {
    grid-row: var(--row);
    grid-column: 3;
    border: 1px dashed rgba(148, 163, 184, 0.25);
    border-radius: 6px;
    background: rgba(15, 23, 42, 0.4);
    display: flex; align-items: center; justify-content: center;
    color: rgba(148, 163, 184, 0.7);
    font-family: var(--solo-font-head); font-size: 0.6rem; font-weight: 700;
    letter-spacing: 0.04em; text-transform: uppercase;
  }

  /* ── Element cell - Atmospheric Neon Vault ──
     Base canvas is an extremely-dark, desaturated charcoal. Every locked
     tile carries a faint group-hue radial as a "ghost hint" of what's
     waiting to be discovered; every unlocked tile lights up the dark
     grid with a real radial bloom + drop-shadow halo. */
  .arc-cell {
    grid-row: var(--row);
    grid-column: var(--col);
    aspect-ratio: 1 / 1;
    background: #1A1D21;
    border: 1px solid rgba(148, 163, 184, 0.10);
    border-radius: 8px;
    padding: 0.3rem 0.35rem 0.25rem;
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    align-items: stretch;
    text-decoration: none;
    color: inherit;
    cursor: default;
    position: relative;
    overflow: hidden;
    user-select: none;
    transition:
      transform .2s ease,
      box-shadow .25s ease,
      border-color .25s ease,
      opacity .25s ease,
      filter .25s ease,
      background .25s ease;
  }
  .arc-cell-num {
    font-family: 'JetBrains Mono', ui-monospace, monospace;
    font-size: 0.58rem;
    font-weight: 500;
    line-height: 1;
    color: rgba(148, 163, 184, 0.7);
    align-self: flex-start;
  }
  .arc-cell-symbol {
    font-family: var(--solo-font-head);
    font-weight: 700;
    font-size: clamp(0.95rem, 1.6vw, 1.25rem);
    line-height: 1;
    letter-spacing: -0.02em;
    color: #F8FAFC;
    text-align: center;
    margin: 0;
    transition: text-shadow .35s ease, color .25s ease;
  }
  .arc-cell-name {
    font-family: var(--solo-font-body);
    font-size: 0.5rem;
    font-weight: 700;
    line-height: 1;
    color: rgba(148, 163, 184, 0.7);
    text-align: center;
    text-transform: uppercase;
    letter-spacing: 0;
    overflow: hidden;
    text-overflow: ellipsis;
    white-space: nowrap;
    /* min-width:0 + max-width:100% lets ellipsis kick in cleanly inside the
       flex column (without these, a long word like BERYLLIUM can push the
       flex item past the cell's content box and clip outside the border). */
    min-width: 0;
    max-width: 100%;
    align-self: stretch;
    box-sizing: border-box;
  }

  /* ── Dormant (locked) - Mystery Mode with group-hue ghost hint ──
     Two background layers: a faint radial of the group's halo colour
     (8% opacity, just enough to whisper "this slot is in the alkali
     family"), and the dark charcoal base underneath. Glyphs stay hidden
     so the actual element identity is still a discovery. */
  .arc-cell.is-locked {
    background:
      radial-gradient(circle at 50% 55%, var(--el-glow, rgba(148,163,184,0.10)) 0%, transparent 70%),
      #1A1D21;
    background-blend-mode: screen, normal;
    border-color: rgba(148, 163, 184, 0.10);
    cursor: not-allowed;
    /* Subtle inner specular - sells "etched glass" */
    box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.03),
                inset 0 -1px 0 rgba(0, 0, 0, 0.4);
  }
  .arc-cell.is-locked:hover {
    /* Lift the ghost hint a touch on hover so the student can feel
       the family connection without revealing the element. */
    background:
      radial-gradient(circle at 50% 50%, var(--el-glow, rgba(148,163,184,0.15)) 0%, transparent 65%),
      #1A1D21;
    border-color: rgba(148, 163, 184, 0.18);
  }
  /* Hide every glyph - undiscovered elements give nothing away */
  .arc-cell.is-locked .arc-cell-num,
  .arc-cell.is-locked .arc-cell-symbol,
  .arc-cell.is-locked .arc-cell-name { visibility: hidden; }

  /* "Next Up" pulse - the level+1 cell. Subtle corner energy node so the
     student knows where the next discovery will land without spoiling it. */
  .arc-cell-pulse {
    position: absolute;
    top: 4px; right: 4px;
    width: 7px; height: 7px;
    border-radius: 50%;
    background: #2DD4BF;
    box-shadow: 0 0 10px rgba(45, 212, 191, 0.65);
    opacity: 0;
    pointer-events: none;
  }
  .arc-cell.is-next-up .arc-cell-pulse {
    opacity: 1;
    animation: arc-next-pulse 2.2s ease-in-out infinite;
  }
  @keyframes arc-next-pulse {
    0%, 100% {
      transform: scale(1);
      box-shadow: 0 0 8px rgba(45, 212, 191, 0.45),
                  0 0 0 0 rgba(45, 212, 191, 0.5);
    }
    50% {
      transform: scale(1.5);
      box-shadow: 0 0 18px rgba(45, 212, 191, 0.85),
                  0 0 0 6px rgba(45, 212, 191, 0.0);
    }
  }
  /* Visible "Lv N" hint pinned to the Next Up tile so even without a
     tooltip a student sees the unlock condition at a glance (Round 2
     persona audit). Hidden on locked-but-not-next cells to keep the
     grid restful; the title attr still carries the same hint. */
  .arc-cell-hint {
    position: absolute;
    bottom: 2px; right: 4px;
    font-family: 'JetBrains Mono', ui-monospace, monospace;
    font-size: 9px; font-weight: 700;
    letter-spacing: .02em;
    color: var(--el-color, #2DD4BF);
    background: rgba(15, 23, 42, 0.62);
    border: 1px solid rgba(45, 212, 191, 0.45);
    border-radius: 4px;
    padding: 1px 4px;
    line-height: 1;
    text-shadow: 0 0 6px rgba(15, 23, 42, 0.55);
    pointer-events: none;
  }

  .arc-cell.is-next-up {
    border-color: rgba(45, 212, 191, 0.30);
    box-shadow: 0 0 0 1px rgba(45, 212, 191, 0.20) inset;
  }

  /* ── Unlocked - atmospheric light-emitting tile ──
     Three layers of glow:
       1. radial-gradient inside the tile (lit-from-within)
       2. inset box-shadow border ring
       3. CSS filter: drop-shadow casts the tile's halo onto the dark grid,
          so the unlocked element literally illuminates its neighbours. */
  .arc-cell.is-unlocked {
    cursor: pointer;
    background:
      radial-gradient(circle at 50% 45%, var(--el-glow, rgba(45,212,191,0.45)) 0%, transparent 70%),
      #1A1D21;
    border-color: var(--el-color, #2DD4BF);
    box-shadow:
      0 0 0 1px var(--el-color, #2DD4BF) inset,
      0 0 18px var(--el-glow, rgba(45, 212, 191, 0.40));
    filter: drop-shadow(0 0 15px var(--el-color, rgba(45, 212, 191, 0.55)));
  }
  .arc-cell.is-unlocked .arc-cell-symbol {
    color: var(--el-color, #2DD4BF);
    text-shadow: 0 0 12px var(--el-glow, rgba(45, 212, 191, 0.50));
  }
  .arc-cell.is-unlocked .arc-cell-num    { color: var(--el-color, #2DD4BF); opacity: 0.85; }
  .arc-cell.is-unlocked .arc-cell-name   { color: var(--el-color, #2DD4BF); opacity: 0.75; }
  .arc-cell.is-unlocked:hover {
    transform: translateY(-3px) scale(1.03);
    box-shadow:
      0 0 0 2px var(--el-color, #2DD4BF) inset,
      0 0 0 4px var(--el-glow, rgba(45, 212, 191, 0.25)),
      0 14px 32px -8px var(--el-glow, rgba(45, 212, 191, 0.65));
    filter: drop-shadow(0 0 22px var(--el-color, rgba(45, 212, 191, 0.75)));
  }
  .arc-cell.is-unlocked:focus-visible {
    outline: 2px solid #2DD4BF;
    outline-offset: 2px;
  }
  /* ── Diagonal shine sweep on hover ──
     A thin specular streak slides across the unlocked tile once per
     hover-enter. Pure CSS via ::after pseudo; the existing
     overflow:hidden on .arc-cell clips the streak. Element bg + halo
     untouched. Reduced-motion users get nothing. */
  .arc-cell.is-unlocked::after {
    content: '';
    position: absolute;
    inset: 0;
    border-radius: 8px;
    background: linear-gradient(
      115deg,
      transparent 30%,
      rgba(255, 255, 255, 0.15) 47%,
      rgba(255, 255, 255, 0.32) 50%,
      rgba(255, 255, 255, 0.15) 53%,
      transparent 70%);
    background-size: 220% 100%;
    background-position: -150% 0;
    background-repeat: no-repeat;
    pointer-events: none;
    opacity: 0;
    transition: opacity .15s ease;
  }
  .arc-cell.is-unlocked:hover::after,
  .arc-cell.is-unlocked:focus-visible::after {
    opacity: 1;
    animation: arc-shine-sweep .9s cubic-bezier(0.22, 1, 0.36, 1) forwards;
  }
  @keyframes arc-shine-sweep {
    from { background-position: -150% 0; }
    to   { background-position:  250% 0; }
  }

  /* ── Click-spin (360° in place) + halo ripple ──
     On click we add .is-clicked from JS for ~520ms. The tile flips
     once around Y (reads as a brisk coin spin from the front) and a
     coloured ring expands from centre and fades. Reduced-motion users
     get a small scale tap instead. */
  .arc-cell.is-unlocked.is-clicked {
    animation: arc-cell-spin .52s cubic-bezier(0.45, 0.05, 0.55, 0.95) both;
  }
  @keyframes arc-cell-spin {
    0%   { transform: translateY(0)    rotateY(0deg)   scale(1); }
    35%  { transform: translateY(-2px) rotateY(180deg) scale(1.06); }
    70%  { transform: translateY(-1px) rotateY(330deg) scale(1.02); }
    100% { transform: translateY(0)    rotateY(360deg) scale(1); }
  }
  .arc-cell.is-unlocked.is-clicked::before {
    content: '';
    position: absolute;
    top: 50%; left: 50%;
    width: 0; height: 0;
    border-radius: 50%;
    border: 2px solid var(--el-color, #2DD4BF);
    box-shadow: 0 0 14px var(--el-glow, rgba(45, 212, 191, 0.55));
    transform: translate(-50%, -50%);
    pointer-events: none;
    opacity: 0;
    animation: arc-cell-ripple .55s ease-out forwards;
    z-index: 1;
  }
  @keyframes arc-cell-ripple {
    0%   { width: 0;    height: 0;    opacity: 0.8; }
    100% { width: 220%; height: 220%; opacity: 0; }
  }
  @media (prefers-reduced-motion: reduce) {
    .arc-cell.is-unlocked::after { display: none; }
    .arc-cell.is-unlocked.is-clicked {
      animation: arc-cell-tap-reduced .18s ease-out both;
    }
    .arc-cell.is-unlocked.is-clicked::before { display: none; }
    @keyframes arc-cell-tap-reduced {
      0%   { transform: scale(1); }
      50%  { transform: scale(0.96); }
      100% { transform: scale(1); }
    }
  }

  /* ── Group palette ── neon-on-dark
     Each entry sets:
       --el-color → the accent / glow tint (vivid)
       --el-bg    → tile fill (deep tinted dark)
       --el-glow  → halo colour for box-shadow + text-shadow
   */
  [data-group="hydrogen"]        { --el-color: #5EEAD4; --el-bg: rgba(45, 212, 191, 0.10);  --el-glow: rgba(45, 212, 191, 0.55); }
  [data-group="alkali"]          { --el-color: #FCA5A5; --el-bg: rgba(248, 113, 113, 0.10);--el-glow: rgba(248, 113, 113, 0.50); }
  [data-group="alkaline-earth"]  { --el-color: #FDBA74; --el-bg: rgba(251, 146, 60, 0.10); --el-glow: rgba(251, 146, 60, 0.50); }
  [data-group="transition"]      { --el-color: var(--solo-mastery, #FCD34D); --el-bg: rgba(245, 158, 11, 0.10); --el-glow: rgba(245, 158, 11, 0.55); }
  [data-group="post-transition"] { --el-color: #BEF264; --el-bg: rgba(132, 204, 22, 0.10); --el-glow: rgba(132, 204, 22, 0.50); }
  [data-group="metalloid"]       { --el-color: #67E8F9; --el-bg: rgba(34, 211, 238, 0.10); --el-glow: rgba(34, 211, 238, 0.55); }
  [data-group="nonmetal"]        { --el-color: #93C5FD; --el-bg: rgba(96, 165, 250, 0.10); --el-glow: rgba(96, 165, 250, 0.55); }
  [data-group="halogen"]         { --el-color: #F9A8D4; --el-bg: rgba(236, 72, 153, 0.10); --el-glow: rgba(236, 72, 153, 0.55); }
  [data-group="noble-gas"]       { --el-color: #C4B5FD; --el-bg: rgba(139, 92, 246, 0.10); --el-glow: rgba(139, 92, 246, 0.60); }
  [data-group="lanthanide"]      { --el-color: #F0ABFC; --el-bg: rgba(217, 70, 239, 0.10); --el-glow: rgba(217, 70, 239, 0.55); }
  [data-group="actinide"]        { --el-color: #FDA4AF; --el-bg: rgba(244, 63, 94, 0.10);  --el-glow: rgba(244, 63, 94, 0.55); }

  /* ── Reaction / "lighting up" animation on newly-unlocked elements ──
     A scale burst with an outer neon-coloured glow that ramps and settles
     into the steady-state group glow. */
  @keyframes arc-reaction-burst {
    0%   { transform: scale(0.92);
           box-shadow: 0 0 0 0 transparent, 0 0 0 0 transparent; }
    35%  { transform: scale(1.20);
           box-shadow: 0 0 0 6px var(--el-glow, rgba(45,212,191,0.55)),
                       0 0 36px 16px var(--el-glow, rgba(45,212,191,0.55)); }
    100% { transform: scale(1.00);
           box-shadow: 0 0 0 1px var(--el-color, #2DD4BF) inset,
                       0 0 16px var(--el-glow, rgba(45,212,191,0.35)); }
  }
  .arc-cell.is-newly-unlocked {
    animation: arc-reaction-burst 1.2s cubic-bezier(0.34, 1.56, 0.64, 1) forwards;
    z-index: 2;
    /* Override the default overflow:hidden on .arc-cell so the
       ::before/::after shockwave rings (inset:-6px + scale up to
       1.85) can spill out of the cell during the burst. */
    overflow: visible;
  }
  /* Particle rings - two delayed shockwaves */
  .arc-cell.is-newly-unlocked::before,
  .arc-cell.is-newly-unlocked::after {
    content: '';
    position: absolute;
    inset: -6px;
    border-radius: inherit;
    pointer-events: none;
    border: 2px solid var(--el-color, #2DD4BF);
    box-shadow: 0 0 18px var(--el-glow, rgba(45,212,191,0.55));
    opacity: 0;
    animation: arc-ring 1.2s cubic-bezier(0.16, 1, 0.3, 1) forwards;
  }
  .arc-cell.is-newly-unlocked::after { animation-delay: 0.22s; }
  @keyframes arc-ring {
    0%   { opacity: 0.8; transform: scale(0.8); }
    100% { opacity: 0;   transform: scale(1.85); }
  }

  /* ── Legend ── */
  .arc-legend {
    display: flex; flex-wrap: wrap; gap: 0.55rem 0.85rem;
    margin-top: 1.5rem; padding: 1rem 1.2rem;
    background: rgba(30, 41, 59, 0.6);
    border: 1px solid rgba(148, 163, 184, 0.15);
    border-radius: 14px;
    backdrop-filter: blur(8px);
  }
  .arc-legend-title {
    font-family: var(--solo-font-head); font-size: 0.62rem; font-weight: 700;
    color: rgba(148, 163, 184, 0.85); letter-spacing: 0.12em; text-transform: uppercase;
    margin-right: 0.75rem; align-self: center;
  }
  .arc-legend-chip {
    display: inline-flex; align-items: center; gap: 0.4rem;
    font-family: var(--solo-font-head); font-size: 0.74rem;
    font-weight: 600; color: #CBD5E1;
    cursor: pointer;
    padding: 2px 6px;
    border-radius: 6px;
    transition: background .15s ease, transform .15s ease;
  }
  .arc-legend-chip:hover,
  .arc-legend-chip:focus-visible {
    background: rgba(148, 163, 184, 0.10);
    outline: none;
    transform: translateY(-1px);
  }
  .arc-legend-chip-swatch {
    width: 12px; height: 12px; border-radius: 3px;
    background: var(--swatch);
    border: 1px solid rgba(255, 255, 255, 0.15);
    box-shadow: 0 0 8px var(--swatch-glow, transparent);
  }

  /* ── Legend → Grid hover sync ──
     When the user hovers/focuses a legend chip, JS sets
     data-hover-group on .arc-grid. The grid then dims every cell
     whose data-group doesn't match, and the matching cells pulse
     gently in their group hue. */
  .arc-grid[data-hover-group] .arc-cell {
    opacity: 0.30;
    filter: grayscale(0.4);
  }
  .arc-grid[data-hover-group] .arc-cell.is-group-pulse {
    opacity: 1;
    filter: none;
    animation: arc-group-pulse 1.6s ease-in-out infinite;
    z-index: 2;
  }
  @keyframes arc-group-pulse {
    0%, 100% {
      transform: scale(1);
      box-shadow:
        0 0 0 1px var(--el-color, #2DD4BF) inset,
        0 0 14px var(--el-glow, rgba(45,212,191,0.40));
    }
    50% {
      transform: scale(1.06);
      box-shadow:
        0 0 0 2px var(--el-color, #2DD4BF) inset,
        0 0 28px var(--el-glow, rgba(45,212,191,0.85));
    }
  }
  /* Pulse on locked cells - intensify the ghost hint without revealing the glyph */
  .arc-grid[data-hover-group] .arc-cell.is-group-pulse.is-locked {
    background:
      radial-gradient(circle at 50% 50%, var(--el-glow, rgba(148,163,184,0.30)) 0%, transparent 60%),
      #1A1D21;
  }

  /* ════════════════════════════════════════════════════════════
     Researcher's Log modal
     ════════════════════════════════════════════════════════════ */
  .arc-modal-backdrop {
    position: fixed; inset: 0; z-index: 998;
    background: rgba(2, 6, 23, 0.72);
    backdrop-filter: blur(6px); -webkit-backdrop-filter: blur(6px);
    opacity: 0; pointer-events: none;
    transition: opacity .25s ease;
    display: flex; align-items: center; justify-content: center;
    padding: 1.5rem;
  }
  .arc-modal-backdrop.is-open { opacity: 1; pointer-events: auto; }

  .arc-modal {
    position: relative;
    background: #1E293B;
    color: #E2E8F0;
    border: 1px solid rgba(148, 163, 184, 0.15);
    border-radius: 22px;
    max-width: 640px; width: 100%;
    max-height: calc(100vh - 3rem);
    overflow-y: auto;
    box-shadow: 0 24px 60px rgba(0, 0, 0, 0.55), 0 0 0 1px rgba(255,255,255,0.03);
    transform: translateY(8px) scale(0.98);
    transition: transform .25s cubic-bezier(0.34,1.56,0.64,1);
  }
  .arc-modal-backdrop.is-open .arc-modal { transform: translateY(0) scale(1); }

  .arc-modal-header {
    display: flex; align-items: center; justify-content: space-between;
    padding: 1rem 1.25rem .85rem;
    border-bottom: 1px solid rgba(148, 163, 184, 0.12);
    background: rgba(15, 23, 42, 0.6);
    border-radius: 22px 22px 0 0;
  }
  .arc-modal-id {
    font-family: 'JetBrains Mono', ui-monospace, monospace;
    font-size: 0.7rem; font-weight: 500;
    color: rgba(148, 163, 184, 0.8); letter-spacing: 0.04em;
    text-transform: uppercase;
  }
  .arc-modal-id strong { color: #F8FAFC; font-weight: 700; }
  .arc-modal-close {
    background: transparent; border: none; cursor: pointer;
    width: 32px; height: 32px; border-radius: 8px;
    display: inline-flex; align-items: center; justify-content: center;
    color: rgba(148, 163, 184, 0.85);
    transition: background .12s ease, color .12s ease;
  }
  .arc-modal-close:hover { background: rgba(148, 163, 184, 0.15); color: #F8FAFC; }
  .arc-modal-close svg { stroke: currentColor; width: 18px; height: 18px; }

  .arc-modal-body { padding: 1.6rem 1.6rem 1.85rem; }

  /* Element tile inside modal */
  .arc-modal-tile {
    display: flex; align-items: stretch; gap: 1rem;
    margin-bottom: 1.4rem;
  }
  .arc-modal-tile-tile {
    width: 96px; height: 96px;
    border-radius: 14px;
    background: var(--el-bg);
    border: 2px solid var(--el-color);
    box-shadow: 0 0 0 1px rgba(255,255,255,0.06) inset,
                0 0 24px var(--el-glow);
    padding: 0.6rem 0.7rem 0.55rem;
    display: flex; flex-direction: column; justify-content: space-between;
    flex-shrink: 0;
  }
  .arc-modal-tile-num {
    font-family: 'JetBrains Mono', ui-monospace, monospace;
    font-size: 0.78rem; font-weight: 500;
    color: var(--el-color); opacity: 0.85;
  }
  .arc-modal-tile-symbol {
    font-family: var(--solo-font-head); font-weight: 700;
    font-size: 2rem; line-height: 1;
    color: var(--el-color);
    text-align: center; letter-spacing: -0.03em;
    text-shadow: 0 0 16px var(--el-glow);
  }
  .arc-modal-tile-name {
    font-family: var(--solo-font-body);
    font-size: 0.6rem; font-weight: 700;
    color: var(--el-color); opacity: 0.85;
    letter-spacing: 0;
    text-transform: uppercase;
    text-align: center;
    overflow: hidden; text-overflow: ellipsis; white-space: nowrap;
    /* Same flex-child clipping protection as .arc-cell-name - lets long
       element names (PRASEODYMIUM, DARMSTADTIUM, etc.) ellipse cleanly
       inside the modal tile rather than pushing past the border. */
    min-width: 0;
    max-width: 100%;
    align-self: stretch;
    box-sizing: border-box;
  }
  .arc-modal-headline { flex: 1; }
  .arc-modal-eyebrow {
    font-family: var(--solo-font-head); font-size: 0.65rem; font-weight: 700;
    color: #5EEAD4; letter-spacing: 0.14em; text-transform: uppercase;
    margin: 0 0 0.4rem;
  }
  .arc-modal-title {
    font-family: var(--solo-font-head); font-size: 1.85rem; font-weight: 700;
    color: #F8FAFC; letter-spacing: -0.02em;
    margin: 0 0 0.45rem; line-height: 1.1;
  }
  .arc-modal-tagline {
    font-family: var(--solo-font-body);
    font-size: 0.95rem; color: #CBD5E1;
    line-height: 1.5; margin: 0;
  }

  /* Properties grid */
  .arc-modal-props {
    display: grid; grid-template-columns: repeat(2, 1fr);
    gap: 0.45rem 1.1rem;
    margin: 0 0 1.5rem; padding: 0.85rem 1rem;
    background: rgba(15, 23, 42, 0.5);
    border-radius: 12px;
    border: 1px solid rgba(148, 163, 184, 0.12);
    font-family: var(--solo-font-body);
  }
  .arc-modal-props dt {
    font-family: var(--solo-font-head); font-size: 0.6rem; font-weight: 700;
    color: rgba(148, 163, 184, 0.85); letter-spacing: 0.12em;
    text-transform: uppercase; margin: 0;
  }
  .arc-modal-props dd {
    font-size: 0.92rem; color: #F8FAFC;
    margin: 0; font-weight: 600;
  }

  /* Researcher's log */
  .arc-modal-log-head {
    display: inline-flex; align-items: center; gap: 0.45rem;
    font-family: var(--solo-font-head); font-size: 0.65rem; font-weight: 700;
    color: rgba(148, 163, 184, 0.85); letter-spacing: 0.14em;
    text-transform: uppercase; margin: 0 0 0.5rem;
  }
  .arc-modal-log-head::before {
    content: ''; width: 14px; height: 1px; background: #2DD4BF;
    box-shadow: 0 0 6px rgba(45,212,191,0.6);
  }
  .arc-modal-h3 {
    font-family: var(--solo-font-head); font-size: 1.15rem; font-weight: 700;
    color: #F8FAFC; letter-spacing: -0.01em;
    margin: 0 0 0.4rem;
  }
  .arc-modal-h3-sub {
    font-family: 'JetBrains Mono', ui-monospace, monospace;
    font-size: 0.78rem; font-weight: 500; color: rgba(148, 163, 184, 0.85);
    margin-left: 0.5rem;
  }
  .arc-modal-log-body {
    font-family: var(--solo-font-body);
    font-size: 0.96rem; line-height: 1.7;
    color: #CBD5E1;
    margin: 0 0 1rem;
  }
  .arc-modal-log-body p { margin: 0 0 0.85rem; }
  .arc-modal-log-body p:last-child { margin-bottom: 0; }

  .arc-modal-footer {
    margin-top: 1.5rem; padding-top: 1.1rem;
    border-top: 1px solid rgba(148, 163, 184, 0.12);
    display: flex; align-items: center; justify-content: space-between;
    flex-wrap: wrap; gap: 0.75rem;
    font-family: var(--solo-font-head);
    font-size: 0.72rem; font-weight: 600;
    color: rgba(148, 163, 184, 0.85); letter-spacing: 0.04em;
  }
  .arc-modal-footer-meta strong { color: #F8FAFC; font-weight: 700; }
  .arc-modal-prev-next { display: flex; gap: 0.4rem; }
  .arc-modal-nav-btn {
    background: transparent; border: 1px solid rgba(148, 163, 184, 0.25);
    border-radius: 999px; padding: 0.4rem 0.75rem;
    font-family: var(--solo-font-head); font-size: 0.78rem; font-weight: 700;
    color: #CBD5E1; cursor: pointer;
    transition: border-color .12s ease, color .12s ease, background .12s ease;
  }
  .arc-modal-nav-btn:hover:not(:disabled) {
    border-color: #2DD4BF;
    color: #5EEAD4;
    background: rgba(45, 212, 191, 0.08);
  }
  .arc-modal-nav-btn:disabled { opacity: 0.4; cursor: not-allowed; }

  /* ── SoloSight gate - public visitors see a blurred teaser ──
     Body gets `.auth-logged-in` from applyAuthBodyClasses() when a
     student is signed in. While that class is absent, the gate
     covers the Vault with a centered Sign-in CTA and the page
     content sits behind a soft blur. */
  /* Default: HIDDEN to stop the gate flashing on every page load for
     signed-in users. We only SHOW it when .auth-logged-out is explicitly
     set on body, or when the preview state opens it deliberately. */
  .arc-gate-overlay {
    position: fixed;
    inset: 0;
    display: none;
    align-items: center;
    justify-content: center;
    background:
      radial-gradient(ellipse 800px 500px at 50% 40%, rgba(8, 16, 31, 0.55) 0%, rgba(8, 16, 31, 0.92) 80%);
    backdrop-filter: blur(14px) saturate(0.85);
    -webkit-backdrop-filter: blur(14px) saturate(0.85);
    z-index: 8500;
    padding: 2rem 1.25rem;
  }
  body.auth-logged-out .arc-gate-overlay { display: flex; }
  /* Hide the overlay's *card* if the user explicitly previews via the
     "Tour the Vault" button (sets is-previewing on body). The blur stays. */
  body.is-vault-preview .arc-gate-overlay { background: rgba(8, 16, 31, 0.18); backdrop-filter: blur(2px); display: flex; }
  body.is-vault-preview .arc-gate-card { display: none; }

  .arc-gate-card {
    max-width: 460px; width: 100%;
    background: linear-gradient(180deg, rgba(15, 23, 42, 0.95) 0%, rgba(8, 16, 31, 0.95) 100%);
    border: 1px solid rgba(45, 212, 191, 0.30);
    border-radius: 18px;
    padding: 2rem 1.75rem 1.75rem;
    text-align: center;
    box-shadow:
      0 30px 60px rgba(0, 0, 0, 0.55),
      0 0 0 4px rgba(45, 212, 191, 0.10),
      0 0 36px rgba(45, 212, 191, 0.18);
  }
  .arc-gate-icon {
    width: 56px; height: 56px;
    margin: 0 auto 1rem;
    border-radius: 14px;
    display: inline-flex; align-items: center; justify-content: center;
    background: rgba(45, 212, 191, 0.10);
    border: 1px solid rgba(45, 212, 191, 0.25);
    color: #5EEAD4;
  }
  .arc-gate-icon svg { width: 28px; height: 28px; stroke: currentColor; fill: none; stroke-width: 2; stroke-linecap: round; stroke-linejoin: round; }
  .arc-gate-eyebrow {
    font-family: var(--solo-font-head);
    font-size: 0.65rem; font-weight: 700;
    letter-spacing: 0.18em; text-transform: uppercase;
    color: rgba(94, 234, 212, 0.85);
    margin: 0 0 0.5rem;
  }
  .arc-gate-card h2 {
    font-family: var(--solo-font-head);
    font-size: 1.4rem; font-weight: 700;
    color: #F8FAFC;
    margin: 0 0 0.75rem;
  }
  .arc-gate-card p {
    font-family: var(--solo-font-body);
    font-size: 0.95rem; line-height: 1.55;
    color: #CBD5E1;
    margin: 0 0 1.4rem;
  }
  .arc-gate-actions {
    display: flex; flex-direction: column; gap: 0.55rem;
  }
  .arc-gate-btn {
    display: inline-flex; align-items: center; justify-content: center;
    padding: 0.7rem 1.1rem;
    border-radius: 999px;
    font-family: var(--solo-font-head);
    font-size: 0.92rem; font-weight: 700;
    letter-spacing: 0.01em;
    cursor: pointer;
    text-decoration: none;
    transition: background .15s ease, transform .12s ease, box-shadow .15s ease;
  }
  .arc-gate-btn-primary {
    background: #2DD4BF; color: #1E293B;
    border: 1px solid #2DD4BF;
    box-shadow: 0 4px 12px rgba(45, 212, 191, 0.25);
  }
  .arc-gate-btn-primary:hover { background: #5EEAD4; transform: translateY(-1px); box-shadow: 0 6px 18px rgba(45, 212, 191, 0.40); }
  .arc-gate-btn-ghost {
    background: transparent; color: #E2E8F0;
    border: 1px solid rgba(148, 163, 184, 0.35);
  }
  .arc-gate-btn-ghost:hover { border-color: rgba(45, 212, 191, 0.55); color: #5EEAD4; }
  .arc-gate-foot {
    font-family: var(--solo-font-body);
    font-size: 0.72rem;
    color: rgba(148, 163, 184, 0.7);
    margin: 1rem 0 0;
  }
  .arc-gate-foot a { color: #5EEAD4; text-decoration: none; }
  .arc-gate-foot a:hover { text-decoration: underline; }

  /* While gated, the underlying content is visually softened so the page
     reads as a real teaser (you can see WHAT it is, but not the detail). */
  body.auth-logged-out .arc-vault-section,
  body.auth-logged-out .arc-hero {
    filter: blur(2.5px) saturate(0.85);
    pointer-events: none;
    user-select: none;
  }
  body.is-vault-preview .arc-vault-section,
  body.is-vault-preview .arc-hero {
    filter: blur(0.5px) saturate(0.95);
  }

  /* ── Mobile · Explorer View ──────────────────────────────────
     Below 720px the 18-column periodic table is horizontally
     scrollable: tiles stay properly tappable (no 12px micro-cells),
     the user pans the layout instead. Scroll-snap clicks each
     period into a comfortable position; a soft fade on the right
     edge hints at off-screen content; iOS gets momentum scrolling. */
  @media (max-width: 720px) {
    .arc-vault-section { padding: 1.5rem 0 3rem; }
    .arc-vault-shell { padding: 0 1rem; }

    .arc-grid-explorer {
      position: relative;
      margin: 0 -1rem;          /* break out of the shell padding */
      padding: 0.5rem 1rem 1rem;
      overflow-x: auto;
      overflow-y: hidden;
      -webkit-overflow-scrolling: touch;
      scroll-snap-type: x proximity;
      scroll-padding-inline: 1rem;
      /* fade the right edge so users see there's more table */
      mask-image: linear-gradient(90deg,
        transparent 0,
        #000 1rem,
        #000 calc(100% - 1.5rem),
        transparent 100%);
    }
    .arc-grid-explorer::-webkit-scrollbar { height: 6px; }
    .arc-grid-explorer::-webkit-scrollbar-thumb { background: rgba(45, 212, 191, 0.35); border-radius: 999px; }

    .arc-grid {
      /* Lock to actual readable cell sizes - 44px touch target.
         Total width = 18 cols * 44 + 17 gaps * 4 ≈ 860px → scrolls. */
      grid-template-columns: repeat(18, 44px);
      grid-template-rows: repeat(7, 44px) 0.5rem repeat(2, 44px);
      gap: 4px;
      width: max-content;       /* natural width, ignore container */
      overflow: visible;        /* parent .arc-grid-explorer scrolls instead */
    }
    .arc-grid > * { scroll-snap-align: start; }

    .arc-cell { padding: 0.22rem 0.25rem 0.2rem; border-radius: 6px; }
    .arc-cell-num { font-size: 0.55rem; }
    .arc-cell-symbol { font-size: 1rem; }
    .arc-cell-name { display: none; }

    /* Central progress card stretches above the (now scrolling) grid */
    .arc-progress { grid-column: 1 / -1; grid-row: auto; padding: 0.55rem 0.75rem; }

    /* Hint pill above the explorer */
    .arc-explorer-hint {
      display: inline-flex; align-items: center; gap: 0.4rem;
      margin: 0.4rem 0 0.6rem;
      padding: 0.35rem 0.7rem;
      background: rgba(15, 23, 42, 0.6);
      border: 1px solid rgba(45, 212, 191, 0.30);
      border-radius: 999px;
      font-family: var(--solo-font-head);
      font-size: 0.7rem; font-weight: 600;
      color: rgba(94, 234, 212, 0.95);
      letter-spacing: 0.04em;
    }
    .arc-explorer-hint svg { width: 12px; height: 12px; stroke: currentColor; fill: none; stroke-width: 2; stroke-linecap: round; stroke-linejoin: round; }

    .arc-modal-tile { flex-direction: column; }
    .arc-modal-tile-tile { width: 80px; height: 80px; }
    .arc-modal-props { grid-template-columns: 1fr; }
  }
  /* Explorer wrapper is hidden above mobile (grid renders inline as before) */
  .arc-grid-explorer { display: contents; }
  @media (max-width: 720px) {
    .arc-grid-explorer { display: block; }
  }
  .arc-explorer-hint { display: none; }
  @media (max-width: 720px) {
    .arc-explorer-hint { display: inline-flex; }
  }