```css
/* ============================================================
   The Entropy Gate — design system
   Obsidian ink, gold, violet. Runic probability codex.
   ============================================================ */

:root {
  --bg: #0a0a0f;
  --bg-2: #101018;
  --panel: rgba(22, 22, 34, 0.55);
  --panel-solid: #12121d;
  --gold: #c9a227;
  --gold-bright: #e8c464;
  --violet: #7c5cff;
  --violet-dim: rgba(124, 92, 255, 0.35);
  --ink-text: #e7dfc8;
  --ink-dim: #a9a28c;
  --line: rgba(201, 162, 39, 0.22);
  --line-strong: rgba(201, 162, 39, 0.45);
  --danger: #e05555;
  --ok: #6fd08c;
  --font-display: "Cinzel", "Cormorant Garamond", "Palatino Linotype", Palatino, "Book Antiqua", Georgia, serif;
  --font-body: "Iowan Old Style", "Palatino Linotype", Palatino, Georgia, serif;
  --font-mono: "SF Mono", "Cascadia Code", "Consolas", "Menlo", monospace;
  --rail-w: 210px;
  --topbar-h: 58px;
}

*, *::before, *::after { box-sizing: border-box; }

html { scroll-behavior: smooth; }
@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
}

body {
  margin: 0;
  background: var(--bg);
  color: var(--ink-text);
  font-family: var(--font-body);
  font-size: 17px;
  line-height: 1.65;
  -webkit-font-smoothing: antialiased;
}

/* Vignette + arcane glow */
body::before {
  content: "";
  position: fixed;
  inset: 0;
  z-index: -2;
  pointer-events: none;
  background:
    radial-gradient(1200px 500px at 50% -8%, rgba(124, 92, 255, 0.10), transparent 60%),
    radial-gradient(1000px 600px at 50% 115%, rgba(201, 162, 39, 0.08), transparent 60%),
    var(--bg);
}

/* Grain — inline SVG turbulence, no external assets */
body::after {
  content: "";
  position: fixed;
  inset: 0;
  z-index: -1;
  pointer-events: none;
  opacity: 0.05;
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='240' height='240'%3E%3Cfilter id='n'%3E%3CfeTurbulence type='fractalNoise' baseFrequency='0.9' numOctaves='2' stitchTiles='stitch'/%3E%3C/filter%3E%3Crect width='240' height='240' filter='url(%23n)'/%3E%3C/svg%3E");
}

::selection { background: rgba(201, 162, 39, 0.35); color: #fff; }

a { color: var(--gold-bright); text-decoration: none; }
a:hover { color: #f2dc9a; }

/* ---------------------------- Top bar ---------------------------- */
.topbar {
  position: sticky;
  top: 0;
  z-index: 50;
  background: rgba(10, 10, 15, 0.92);
  backdrop-filter: blur(8px);
  border-bottom: 1px solid var(--line);
}
.topbar-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  padding: 0.55rem 1rem;
  max-width: 1500px;
  margin: 0 auto;
}
.brand { display: flex; align-items: baseline; gap: 0.5rem; }
.brand-rune { color: var(--gold); font-size: 1.2rem; }
.brand-name {
  font-family: var(--font-display);
  letter-spacing: 0.08em;
  font-size: 1rem;
  color: var(--ink-text);
}
.meter { display: flex; align-items: baseline; gap: 0.6rem; }
.meter-label { font-size: 0.75rem; color: var(--ink-dim); letter-spacing: 0.06em; text-transform: uppercase; }
.meter-count {
  font-family: var(--font-mono);
  color: var(--gold-bright);
  border: 1px solid var(--line-strong);
  border-radius: 4px;
  padding: 0.1rem 0.5rem;
  font-size: 0.85rem;
}
.meter-runes { color: var(--ink-dim); font-size: 0.85rem; letter-spacing: 0.2em; }
.meter-runes .lit { color: var(--gold-bright); }

.nav-mobile { display: none; }

/* ---------------------------- Rail nav ---------------------------- */
.nav-rail {
  position: fixed;
  top: var(--topbar-h);
  left: 0;
  width: var(--rail-w);
  padding: 1.2rem 0.8rem;
  display: flex;
  flex-direction: column;
  gap: 0.25rem;
  z-index: 40;
}
.rail-title {
  font-family: var(--font-display);
  font-size: 0.8rem;
  letter-spacing: 0.25em;
  color: var(--ink-dim);
  text-transform: uppercase;
  padding: 0 0.6rem 0.7rem;
}
.nav-link {
  display: flex;
  align-items: center;
  gap: 0.65rem;
  padding: 0.45rem 0.6rem;
  border-radius: 6px;
  color: var(--ink-dim);
  border: 1px solid transparent;
  transition: color 0.2s, border-color 0.2s, background 0.2s;
}
.nav-link .rune { font-size: 1.1rem; width: 1.4rem; text-align: center; color: var(--gold); }
.nav-link:hover { color: var(--ink-text); background: rgba(201, 162, 39, 0.06); }
.nav-link.active {
  color: var(--ink-text);
  border-color: var(--line);
  background: rgba(201, 162, 39, 0.08);
}
.nav-link.measured .rune {
  color: var(--gold-bright);
  text-shadow: 0 0 8px rgba(232, 196, 100, 0.8);
}
.rail-meter {
  margin-top: 1.4rem;
  padding: 0.8rem 0.6rem;
  border-top: 1px solid var(--line);
}
.rail-meter-label { font-size: 0.7rem; color: var(--ink-dim); letter-spacing: 0.04em; margin-bottom: 0.3rem; }
.rail-meter-count {
  font-family: var(--font-mono);
  font-size: 1.4rem;
  color: var(--gold-bright);
}

/* ---------------------------- Main layout ---------------------------- */
main { margin-left: var(--rail-w); }

/* ---------------------------- Hero ---------------------------- */
.hero {
  position: relative;
  min-height: calc(100vh - var(--topbar-h));
  display: flex;
  align-items: center;
  justify-content: center;
  overflow: hidden;
  border-bottom: 1px solid var(--line);
}
.hero-canvas-wrap { position: absolute; inset: 0; }
#hero-canvas { width: 100%; height: 100%; display: block; }
.hero-copy {
  position: relative;
  z-index: 2;
  text-align: center;
  padding: 2rem 1.5rem;
  max-width: 760px;
}
.hero-runes { color: var(--gold); letter-spacing: 0.6em; font-size: 0.95rem; margin-bottom: 1.2rem; opacity: 0.9; }
.hero-title {
  font-family: var(--font-display);
  font-weight: 400;
  font-size: clamp(2.6rem, 7vw, 4.6rem);
  letter-spacing: 0.1em;
  margin: 0 0 0.7rem;
  text-shadow: 0 0 40px rgba(124, 92, 255, 0.35);
}
.hero-sub {
  font-size: clamp(1rem, 2.2vw, 1.25rem);
  color: var(--ink-dim);
  margin: 0 0 1.4rem;
}
.hero-motto {
  font-family: var(--font-mono);
  font-size: 0.8rem;
  letter-spacing: 0.2em;
  color: var(--gold);
  border: 1px solid var(--line);
  display: inline-block;
  padding: 0.4rem 0.9rem;
  border-radius: 3px;
  margin-bottom: 2rem;
}
.hero-cta {
  display: inline-block;
  font-family: var(--font-display);
  letter-spacing: 0.12em;
  padding: 0.7rem 1.6rem;
  border: 1px solid var(--line-strong);
  border-radius: 4px;
  color: var(--gold-bright);
  background: rgba(201, 162, 39, 0.07);
  transition: box-shadow 0.25s, background 0.25s;
}
.hero-cta:hover {
  background: rgba(201, 162, 39, 0.14);
  box-shadow: 0 0 24px rgba(201, 162, 39, 0.25);
  color: #f2dc9a;
}

/* ---------------------------- Chapters ---------------------------- */
.chapter {
  max-width: 1080px;
  margin: 0 auto;
  padding: 4.5rem 1.4rem 3rem;
  border-bottom: 1px solid rgba(201, 162, 39, 0.10);
}
.chapter-head {
  display: flex;
  align-items: center;
  gap: 1.2rem;
  margin-bottom: 1.6rem;
}
.chapter-rune {
  font-size: 3.2rem;
  color: var(--gold);
  text-shadow: 0 0 26px rgba(201, 162, 39, 0.45);
  line-height: 1;
}
.chapter-kicker {
  font-family: var(--font-mono);
  font-size: 0.75rem;
  letter-spacing: 0.25em;
  color: var(--ink-dim);
  margin: 0 0 0.2rem;
  text-transform: uppercase;
}
.chapter h2 {
  font-family: var(--font-display);
  font-weight: 400;
  font-size: clamp(1.5rem, 3.4vw, 2.2rem);
  letter-spacing: 0.05em;
  margin: 0;
}
.chapter p { max-width: 78ch; }
.chapter strong { color: var(--gold-bright); font-weight: 600; }
.chapter em { color: var(--ink-text); }

.formula-line {
  font-family: var(--font-body);
  font-size: 1.12rem;
  color: var(--ink-text);
  background: rgba(124, 92, 255, 0.05);
  border: 1px solid rgba(124, 92, 255, 0.18);
  border-radius: 6px;
  padding: 0.7rem 1rem;
  display: block;
  overflow-x: auto;
  white-space: nowrap;
}

/* CSS fraction component */
.frac {
  display: inline-flex;
  flex-direction: column;
  vertical-align: middle;
  text-align: center;
  margin: 0 0.2em;
  line-height: 1.25;
}
.frac .num { padding: 0 0.35em; border-bottom: 1px solid currentColor; }
.frac .den { padding: 0 0.35em; }

/* ---------------------------- Theorem boxes ---------------------------- */
.theorem {
  position: relative;
  margin: 1.4rem 0;
  padding: 1.1rem 1.3rem 1.1rem 1.5rem;
  background: var(--panel);
  border: 1px solid var(--line);
  border-left: 3px solid var(--gold);
  border-radius: 6px;
}
.theorem-tag {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  font-family: var(--font-display);
  letter-spacing: 0.08em;
  font-size: 0.85rem;
  color: var(--gold-bright);
  margin-bottom: 0.5rem;
}
.theorem-tag .rune { color: var(--violet); }
.theorem p:last-child { margin-bottom: 0; }
.theorem ol, .theorem ul { margin: 0.4rem 0 0.4rem 1.2rem; }
.def-list li { margin-bottom: 0.5rem; }

/* ---------------------------- Simulations ---------------------------- */
.sim {
  margin: 2.2rem 0;
  padding: 1.4rem;
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: 10px;
  box-shadow: 0 10px 40px rgba(0, 0, 0, 0.35);
}
.sim-head h4 {
  font-family: var(--font-display);
  font-weight: 400;
  letter-spacing: 0.06em;
  font-size: 1.15rem;
  margin: 0 0 0.25rem;
  color: var(--gold-bright);
}
.sim-head .rune { color: var(--violet); margin-right: 0.4rem; }
.sim-sub { color: var(--ink-dim); font-size: 0.95rem; margin: 0 0 1rem; }
.sim-note { color: var(--ink-dim); font-size: 0.88rem; margin: 0.8rem 0 0; }

.sim-controls {
  display: flex;
  flex-wrap: wrap;
  gap: 0.9rem 1.4rem;
  align-items: flex-end;
  margin-bottom: 1rem;
}
.ctl { display: flex; flex-direction: column; gap: 0.3rem; min-width: 140px; }
.ctl-label { font-size: 0.78rem; color: var(--ink-dim); letter-spacing: 0.04em; }
.ctl output { font-family: var(--font-mono); color: var(--gold-bright); font-size: 0.9rem; }
.ctl-check { flex-direction: row; align-items: center; gap: 0.5rem; min-width: unset; }
.ctl-check span { font-size: 0.85rem; color: var(--ink-dim); }

.sim input[type="range"] {
  accent-color: var(--gold);
  width: 170px;
  cursor: pointer;
}
.ctl select,
.sim input[type="number"] {
  background: var(--panel-solid);
  color: var(--ink-text);
  border: 1px solid var(--line);
  border-radius: 5px;
  padding: 0.35rem 0.5rem;
  font-family: var(--font-body);
  font-size: 0.9rem;
}
.ctl select { cursor: pointer; }
.sim input[type="number"] { width: 90px; text-align: right; font-family: var(--font-mono); }
.sim input:focus-visible,
.ctl select:focus-visible,
.btn:focus-visible,
a:focus-visible {
  outline: 2px solid var(--violet);
  outline-offset: 2px;
}

.btns { flex-direction: row; gap: 0.5rem; }
.btn {
  font-family: var(--font-display);
  letter-spacing: 0.08em;
  font-size: 0.8rem;
  color: var(--gold-bright);
  background: rgba(201, 162, 39, 0.08);
  border: 1px solid var(--line-strong);
  border-radius: 5px;
  padding: 0.45rem 0.9rem;
  cursor: pointer;
  transition: box-shadow 0.2s, background 0.2s, color 0.2s;
}
.btn:hover { background: rgba(201, 162, 39, 0.16); box-shadow: 0 0 18px rgba(201, 162, 39, 0.25); }
.btn:active { transform: translateY(1px); }
.btn-ghost { color: var(--ink-dim); border-color: var(--line); background: transparent; }
.btn-ghost:hover { color: var(--ink-text); }
.btn[aria-pressed="true"] { background: rgba(201, 162, 39, 0.22); }

.sim-readout {
  display: flex;
  flex-wrap: wrap;
  gap: 0.7rem 1.6rem;
  margin-bottom: 1rem;
}
.stat { display: flex; flex-direction: column; }
.stat-label { font-size: 0.72rem; color: var(--ink-dim); letter-spacing: 0.05em; text-transform: uppercase; }
.stat-value, .ctl output {
  font-family: var(--font-mono);
  font-size: 1rem;
  color: var(--ink-text);
}
.stat.big .stat-value { color: var(--gold-bright); font-size: 1.25rem; }

.verdict {
  font-family: var(--font-mono);
  font-size: 0.85rem;
  padding: 0.35rem 0.6rem;
  border: 1px solid var(--line);
  border-radius: 5px;
  color: var(--ink-dim);
}
.verdict.ok { color: var(--ok); border-color: rgba(111, 208, 140, 0.4); }
.verdict.bad { color: var(--danger); border-color: rgba(224, 85, 85, 0.4); }
.verdict.gold { color: var(--gold-bright); border-color: var(--line-strong); }

.sim-stage { margin-top: 0.4rem; }
.sim-stage.two-col {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 1.2rem;
}
.chart-figure { margin: 0; min-width: 0; }
.chart-figure figcaption {
  font-size: 0.75rem;
  color: var(--ink-dim);
  letter-spacing: 0.05em;
  text-transform: uppercase;
  margin-bottom: 0.35rem;
}
.sim canvas {
  display: block;
  width: 100%;
  background: rgba(8, 8, 13, 0.6);
  border: 1px solid rgba(201, 162, 39, 0.14);
  border-radius: 6px;
}

/* Legends */
.legend-dot { display: inline-block; width: 0.7em; height: 0.7em; border-radius: 2px; margin: 0 0.25em 0 0.6em; vertical-align: -0.05em; }
.legend-dot.tp { background: var(--gold-bright); }
.legend-dot.fp { background: var(--violet); }
.legend-dot.fn { background: var(--danger); }
.legend-dot.tn { background: #4a4a5e; }
.legend-sq { display: inline-block; width: 0.75em; height: 0.75em; margin: 0 0.2em; vertical-align: -0.08em; }
.legend-sq.h { background: var(--gold-bright); }
.legend-sq.t { background: var(--violet); }

/* Enumerated sample-space cards */
.cards {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(64px, 1fr));
  gap: 0.45rem;
  margin-top: 0.5rem;
}
.cards .card {
  font-family: var(--font-mono);
  font-size: 0.85rem;
  text-align: center;
  padding: 0.4rem 0.2rem;
  border: 1px solid rgba(124, 92, 255, 0.25);
  border-radius: 4px;
  color: var(--ink-dim);
  background: rgba(124, 92, 255, 0.04);
  transition: border-color 0.15s, background 0.15s, color 0.15s;
}
.cards .card.hit {
  color: var(--gold-bright);
  border-color: var(--line-strong);
  background: rgba(201, 162, 39, 0.12);
  box-shadow: 0 0 12px rgba(201, 162, 39, 0.25);
}

/* Combinatorics grid */
.combo-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
  gap: 0.9rem;
}
.combo-cell {
  border: 1px solid var(--line);
  border-radius: 6px;
  padding: 0.8rem 1rem;
  background: rgba(8, 8, 13, 0.5);
}
.combo-formula { font-size: 0.9rem; color: var(--ink-text); margin-bottom: 0.3rem; }
.combo-label { font-size: 0.72rem; color: var(--ink-dim); text-transform: uppercase; letter-spacing: 0.05em; }
.combo-value {
  font-family: var(--font-mono);
  font-size: 1.05rem;
  color: var(--gold-bright);
  word-break: break-all;
  margin-top: 0.25rem;
}

/* Conditional probability table */
.cp-layout {
  display: grid;
  grid-template-columns: minmax(280px, 1.2fr) 1fr;
  gap: 1.4rem;
  align-items: start;
}
.cp-table { border-collapse: collapse; }
.cp-table th {
  font-family: var(--font-display);
  font-weight: 400;
  font-size: 0.85rem;
  color: var(--ink-dim);
  padding: 0.4rem 0.6rem;
  text-align: center;
}
.cp-table th[scope="row"] { text-align: right; }
.cp-table td { border: 1px solid var(--line); padding: 0.3rem; }
.cp-readout { display: flex; flex-direction: column; gap: 0.5rem; }

/* House edge table */
.edge-table {
  width: 100%;
  border-collapse: collapse;
  margin-top: 0.6rem;
  font-family: var(--font-mono);
  font-size: 0.85rem;
}
.edge-table th {
  font-family: var(--font-display);
  font-weight: 400;
  letter-spacing: 0.06em;
  font-size: 0.75rem;
  color: var(--gold);
  border-bottom: 1px solid var(--line-strong);
  padding: 0.4rem 0.5rem;
  text-align: left;
}
.edge-table td {
  border-bottom: 1px solid rgba(201, 162, 39, 0.12);
  padding: 0.4rem 0.5rem;
  color: var(--ink-text);
}
.edge-table tr:last-child td { border-bottom: none; }
.edge-table .neg { color: var(--danger); }

/* Streak strip */
.streak {
  display: flex;
  flex-wrap: wrap;
  gap: 3px;
  min-height: 42px;
  padding: 0.6rem;
  border: 1px solid rgba(124, 92, 255, 0.2);
  border-radius: 6px;
  background: rgba(8, 8, 13, 0.5);
  margin-bottom: 1rem;
}
.streak span {
  width: 12px;
  height: 12px;
  border-radius: 2px;
}
.streak span.h { background: var(--gold-bright); }
.streak span.t { background: var(--violet); }

/* Quiz */
.quiz {
  border: 1px solid rgba(124, 92, 255, 0.2);
  border-radius: 8px;
  padding: 1rem 1.2rem;
  background: rgba(124, 92, 255, 0.04);
}
.quiz-q { margin: 0 0 0.7rem; }
.quiz-opts { display: flex; flex-wrap: wrap; gap: 0.5rem; }
.quiz-out { margin: 0.7rem 0 0; font-family: var(--font-mono); font-size: 0.88rem; color: var(--ink-dim); }
.quiz-out.right { color: var(--ok); }
.quiz-out.wrong { color: var(--danger); }

/* Footer */
.footer {
  max-width: 1080px;
  margin: 0 auto;
  padding: 3.5rem 1.4rem 4.5rem;
  color: var(--ink-dim);
  font-size: 0.9rem;
  text-align: center;
}
.footer-epigraph {
  font-family: var(--font-display);
  letter-spacing: 0.2em;
  color: var(--gold);
  margin-bottom: 1rem;
}
.footer-runes { letter-spacing: 0.5em; color: var(--gold); opacity: 0.6; margin-top: 1.2rem; }

/* ---------------------------- Responsive ---------------------------- */
@media (max-width: 900px) {
  main { margin-left: 0; }
  .nav-rail { display: none; }
  .nav-mobile {
    display: flex;
    overflow-x: auto;
    gap: 0.4rem;
    padding: 0.4rem 0.8rem 0.5rem;
  }
  .nav-mobile .nav-link { white-space: nowrap; font-size: 0.78rem; padding: 0.35rem 0.55rem; }
  .nav-mobile .nav-link .rail-text { display: inline; }
  .topbar-inner { flex-wrap: wrap; gap: 0.4rem; }
  :root { --topbar-h: 100px; }
  .sim-stage.two-col { grid-template-columns: 1fr; }
  .cp-layout { grid-template-columns: 1fr; }
  .chapter { padding: 3rem 1rem 2rem; }
  .hero { min-height: calc(100vh - var(--topbar-h)); }
}

@media (max-width: 560px) {
  .formula-line { font-size: 0.98rem; }
  .chapter-head { gap: 0.8rem; }
  .chapter-rune { font-size: 2.2rem; }
  .sim { padding: 1rem; }
  .sim input[type="range"] { width: 100%; max-width: 220px; }
}

/* Reduced motion: static codex — RAF-driven sims require explicit start anyway */
@media (prefers-reduced-motion: reduce) {
  .btn, .nav-link, .cards .card, .hero-cta { transition: none; }
}
```