/* Reinforcement Learning — concept guide.
   Inherits cho.books "old bookshop" tone; adds a teaching layer:
   concept cards, equation blocks, motion stages, sidebar TOC. */

:root {
  --paper:     oklch(94% 0.014 80);
  --paper-2:   oklch(91% 0.018 75);
  --cream:     oklch(97% 0.012 85);
  --ink:       oklch(22% 0.020 55);
  --ink-soft:  oklch(22% 0.020 55 / 0.65);
  --muted:     oklch(40% 0.020 55);
  --rule:      oklch(45% 0.025 50);
  --hairline:  oklch(78% 0.015 70);
  --accent:    oklch(45% 0.13 35);          /* deep red */
  --accent-soft: oklch(45% 0.13 35 / 0.10);
  --gold:      oklch(72% 0.13 78);
  --gold-soft: oklch(72% 0.13 78 / 0.20);
  --olive:     oklch(50% 0.07 110);
  --teal:      oklch(45% 0.07 200);
  --plum:      oklch(40% 0.10 340);
  --blue:      oklch(48% 0.10 250);

  /* teaching-state semantic colors (used by motion + diagrams) */
  --c-agent:   oklch(45% 0.13 35);
  --c-value-hi:oklch(50% 0.13 145);
  --c-value-lo:oklch(60% 0.12 30);
  --c-reward:  oklch(72% 0.13 78);
  --c-td:      oklch(48% 0.10 250);
  --c-wall:    oklch(30% 0.020 55);

  --font-display: 'Iowan Old Style', 'Charter', 'Source Serif Pro', Georgia, 'Noto Serif KR', serif;
  --font-body:    'Iowan Old Style', 'Charter', 'Source Serif Pro', Georgia, 'Noto Serif KR', serif;
  --font-sans:    -apple-system, BlinkMacSystemFont, 'Segoe UI', system-ui, sans-serif;
  --font-mono:    'JetBrains Mono', 'IBM Plex Mono', ui-monospace, Menlo, monospace;

  --maxw: 1180px;
  --maxw-read: 720px;
  --pad-x: clamp(20px, 4vw, 56px);
}

*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

html, body {
  background: var(--paper);
  color: var(--ink);
  font-family: var(--font-body);
  font-size: 17px;
  line-height: 1.62;
  -webkit-font-smoothing: antialiased;
  text-rendering: optimizeLegibility;
}

body::before {
  content: '';
  position: fixed;
  inset: 0;
  pointer-events: none;
  background-image: radial-gradient(oklch(22% 0.020 55 / 0.04) 1px, transparent 1px);
  background-size: 3px 3px;
  z-index: 0;
}
body > * { position: relative; z-index: 1; }

a { color: var(--ink); text-decoration: none; }
a:hover { color: var(--accent); }

h1, h2, h3, h4, .display {
  font-family: var(--font-display);
  font-weight: 500;
  letter-spacing: -0.005em;
  line-height: 1.2;
  color: var(--ink);
}

.container {
  max-width: var(--maxw);
  margin: 0 auto;
  padding: 0 var(--pad-x);
}

/* ───────── Masthead ───────── */
.masthead {
  border-top: 5px solid var(--ink);
  border-bottom: 1px solid var(--hairline);
  background: var(--cream);
}
.mast-inner {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: 24px;
  padding: 16px 0 13px;
}
.mast-inner .brand {
  font-family: var(--font-display);
  font-size: 21px;
}
.mast-inner .brand small {
  font-family: var(--font-mono);
  font-size: 10.5px;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--muted);
  margin-left: 10px;
}
.mast-meta {
  display: flex;
  gap: 18px;
  font-family: var(--font-mono);
  font-size: 11px;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--muted);
}

/* ───────── Eyebrow / kicker ───────── */
.eyebrow {
  font-family: var(--font-mono);
  font-size: 11px;
  letter-spacing: 0.16em;
  text-transform: uppercase;
  color: var(--accent);
}

/* ───────── Chapter hero ───────── */
.ch-hero {
  padding: 54px 0 32px;
  border-bottom: 1px solid var(--hairline);
}
.ch-hero .eyebrow { display: block; margin-bottom: 16px; }
.ch-hero h1 {
  font-size: clamp(34px, 5vw, 58px);
  line-height: 1.05;
  letter-spacing: -0.02em;
  max-width: 18ch;
}
.ch-hero .standfirst {
  margin-top: 20px;
  font-size: clamp(18px, 1.8vw, 22px);
  font-style: italic;
  color: var(--muted);
  max-width: 60ch;
  line-height: 1.5;
}
.ch-hero .hero-tags {
  margin-top: 22px;
  display: flex;
  flex-wrap: wrap;
  gap: 9px;
}
.tag {
  font-family: var(--font-mono);
  font-size: 10.5px;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
  border: 1px solid var(--hairline);
  background: var(--cream);
  padding: 4px 9px;
  border-radius: 2px;
}
.tag.accent { color: var(--accent); border-color: var(--accent); background: var(--accent-soft); }

/* ───────── Reading layout (article + sidebar) ───────── */
.ch-layout {
  display: grid;
  grid-template-columns: 1fr minmax(0, var(--maxw-read)) 1fr;
  gap: 0;
  padding: 8px 0 0;
}
.ch-toc {
  position: sticky;
  top: 20px;
  align-self: start;
  padding: 32px 24px 0 0;
  font-family: var(--font-mono);
  font-size: 11px;
  line-height: 2;
}
.ch-toc .toc-label {
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--muted);
  display: block;
  margin-bottom: 6px;
}
.ch-toc a { color: var(--ink-soft); display: block; }
.ch-toc a:hover { color: var(--accent); }
.ch-article { min-width: 0; }
.ch-rail { } /* right spacer */

@media (max-width: 1080px) {
  .ch-layout { grid-template-columns: 1fr; }
  .ch-toc { position: static; padding: 0 0 20px; }
}

/* ───────── Section ───────── */
.section {
  padding: 36px 0;
  border-top: 1px solid var(--hairline);
}
.section:first-of-type { border-top: 0; }
.section-title {
  font-size: clamp(22px, 2.4vw, 30px);
  letter-spacing: -0.01em;
  margin-bottom: 16px;
}
.section-title .num {
  font-family: var(--font-mono);
  font-size: 11px;
  letter-spacing: 0.16em;
  text-transform: uppercase;
  color: var(--accent);
  display: block;
  margin-bottom: 6px;
}

/* ───────── Prose ───────── */
.prose { max-width: 64ch; }
.prose p { margin: 0 0 15px; line-height: 1.66; }
.prose p:last-child { margin-bottom: 0; }
.prose strong { font-weight: 600; }
.prose em { font-style: italic; }
.prose ul, .prose ol { margin: 0 0 15px; padding-left: 24px; }
.prose li { margin: 0 0 7px; }
.prose code {
  font-family: var(--font-mono);
  font-size: 0.88em;
  background: oklch(22% 0.020 55 / 0.07);
  padding: 1px 5px;
  border-radius: 2px;
}
.prose .lede {
  font-size: clamp(19px, 1.9vw, 23px);
  line-height: 1.5;
  color: var(--ink);
}
.prose h3 {
  font-size: 20px;
  margin: 26px 0 9px;
}

/* ───────── Concept card ───────── */
.concept {
  margin: 24px 0;
  border: 1px solid var(--hairline);
  background: var(--cream);
  border-left: 3px solid var(--accent);
}
.concept .c-head {
  display: flex;
  align-items: baseline;
  gap: 10px;
  padding: 10px 16px;
  border-bottom: 1px solid var(--hairline);
}
.concept .c-tag {
  font-family: var(--font-mono);
  font-size: 10px;
  letter-spacing: 0.13em;
  text-transform: uppercase;
  color: var(--accent);
  font-weight: 600;
}
.concept .c-name {
  font-family: var(--font-display);
  font-size: 16px;
}
.concept .c-body {
  padding: 14px 16px;
  font-size: 15px;
  line-height: 1.62;
}
.concept .c-body p { margin: 0 0 9px; }
.concept .c-body p:last-child { margin-bottom: 0; }

/* intuition / pitfall / key callouts */
.note {
  margin: 22px 0;
  padding: 13px 16px 13px 17px;
  border-left: 3px solid var(--rule);
  background: var(--paper-2);
  font-size: 15px;
  line-height: 1.6;
}
.note .n-label {
  display: block;
  font-family: var(--font-mono);
  font-size: 10px;
  letter-spacing: 0.13em;
  text-transform: uppercase;
  font-weight: 600;
  margin-bottom: 5px;
}
.note.intuition { border-color: var(--accent); }
.note.intuition .n-label { color: var(--accent); }
.note.pitfall { border-color: var(--c-value-lo); background: oklch(60% 0.12 30 / 0.07); }
.note.pitfall .n-label { color: var(--c-value-lo); }
.note.key { border-color: var(--gold); background: var(--gold-soft); }
.note.key .n-label { color: oklch(55% 0.13 78); }

/* ───────── Equation block ───────── */
.equation {
  margin: 22px 0;
  background: var(--cream);
  border: 1px solid var(--hairline);
  border-top: 2px solid var(--ink);
  padding: 0;
}
.equation .eq-render {
  padding: 20px 24px;
  font-family: 'IBM Plex Mono', var(--font-mono);
  font-size: clamp(15px, 2vw, 19px);
  text-align: center;
  line-height: 1.8;
  overflow-x: auto;
}
.equation .eq-where {
  border-top: 1px solid var(--hairline);
  padding: 11px 16px;
  font-size: 13px;
  line-height: 1.6;
  color: var(--muted);
  background: var(--paper-2);
}
.equation .eq-where b {
  font-family: 'IBM Plex Mono', var(--font-mono);
  color: var(--ink);
  font-weight: 600;
}
.equation .eq-name {
  font-family: var(--font-mono);
  font-size: 10px;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--accent);
  padding: 7px 16px 0;
}
.eq-inline {
  font-family: 'IBM Plex Mono', var(--font-mono);
  font-size: 0.94em;
}

/* ───────── Motion stage ───────── */
.stage {
  margin: 24px 0;
  border: 1px solid var(--hairline);
  background: var(--cream);
}
.stage-head {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 14px;
  padding: 10px 14px;
  border-bottom: 1px solid var(--hairline);
  flex-wrap: wrap;
}
.stage-head .caption {
  font-family: var(--font-mono);
  font-size: 11px;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
}
.stage-head .counter {
  font-family: var(--font-mono);
  font-size: 11px;
  color: var(--muted);
}
.stage-head .counter b { color: var(--ink); }
.stage-canvas {
  padding: 18px;
  display: flex;
  align-items: center;
  justify-content: center;
  background:
    radial-gradient(circle at 15px 15px, oklch(22% 0.020 55 / 0.05) 1px, transparent 1.5px) 0 0 / 22px 22px,
    var(--cream);
  min-height: 360px;
}
.stage-canvas svg { width: 100%; height: auto; max-width: 900px; display: block; }
.stage-narrate {
  padding: 13px 16px;
  border-top: 1px solid var(--hairline);
  font-style: italic;
  font-size: 14.5px;
  line-height: 1.55;
  min-height: 52px;
}
.stage-narrate .s-tag {
  font-style: normal;
  font-family: var(--font-mono);
  font-size: 10px;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--accent);
  margin-right: 8px;
}
.stage-foot {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  padding: 10px 14px;
  border-top: 1px solid var(--hairline);
  flex-wrap: wrap;
}
.stage-controls { display: flex; gap: 8px; align-items: center; }
.btn {
  font-family: var(--font-sans);
  font-size: 13px;
  font-weight: 600;
  border: 1px solid var(--ink);
  background: var(--cream);
  color: var(--ink);
  padding: 7px 13px;
  border-radius: 2px;
  cursor: pointer;
  transition: background 90ms ease, color 90ms ease;
}
.btn:hover { background: var(--ink); color: var(--cream); }
.btn.primary { background: var(--ink); color: var(--cream); }
.btn.primary:hover { background: var(--accent); border-color: var(--accent); }
.btn:disabled { opacity: 0.4; cursor: not-allowed; }
.stage-hint {
  font-family: var(--font-mono);
  font-size: 10.5px;
  color: var(--muted);
  letter-spacing: 0.04em;
}

/* ───────── Algorithm pseudo block ───────── */
.algo {
  margin: 22px 0;
  border: 1px solid var(--hairline);
  background: oklch(24% 0.018 55);
  overflow: hidden;
}
.algo .algo-head {
  padding: 8px 14px;
  border-bottom: 1px solid oklch(34% 0.018 55);
  font-family: var(--font-mono);
  font-size: 10.5px;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: oklch(72% 0.04 78);
}
.algo pre {
  margin: 0;
  padding: 14px 16px;
  overflow-x: auto;
  font-family: var(--font-mono);
  font-size: 12.5px;
  line-height: 1.65;
  color: oklch(88% 0.015 80);
}
.algo .kw  { color: oklch(74% 0.12 25); }
.algo .op  { color: oklch(78% 0.10 250); }
.algo .cm  { color: oklch(56% 0.03 120); font-style: italic; }
.algo .em  { color: oklch(80% 0.12 78); }

/* ───────── Comparison table ───────── */
.rtable {
  width: 100%;
  border-collapse: collapse;
  font-size: 14px;
  margin: 20px 0;
}
.rtable caption {
  text-align: left;
  font-family: var(--font-mono);
  font-size: 10.5px;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--muted);
  padding-bottom: 8px;
}
.rtable th, .rtable td {
  text-align: left;
  padding: 9px 12px;
  border-bottom: 1px solid var(--hairline);
  vertical-align: top;
}
.rtable thead th {
  font-family: var(--font-mono);
  font-size: 10px;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
  border-bottom: 1.5px solid var(--rule);
}
.rtable tbody th { font-weight: 600; }

/* ───────── Chapter nav ───────── */
.ch-nav {
  display: flex;
  justify-content: space-between;
  gap: 16px;
  margin-top: 40px;
  padding: 22px 0;
  border-top: 1px solid var(--hairline);
  flex-wrap: wrap;
}
.ch-nav a {
  font-family: var(--font-mono);
  font-size: 11px;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
}
.ch-nav a:hover { color: var(--accent); }
.ch-nav a b { display: block; font-family: var(--font-display); font-size: 15px; text-transform: none; letter-spacing: 0; color: var(--ink); margin-top: 3px; }

/* ───────── Footer ───────── */
.colophon {
  border-top: 1px solid var(--hairline);
  margin-top: 40px;
  padding: 26px 0 50px;
  font-family: var(--font-mono);
  font-size: 11px;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
  display: flex;
  justify-content: space-between;
  gap: 20px;
  flex-wrap: wrap;
}

/* ───────── Cross-reference chips ───────── */
.xref {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin: 18px 0;
}
.xref a {
  font-family: var(--font-mono);
  font-size: 11px;
  border: 1px solid var(--hairline);
  background: var(--cream);
  padding: 6px 11px;
  transition: border-color 100ms ease;
}
.xref a:hover { border-color: var(--ink); }
.xref a .xk {
  color: var(--accent);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  font-size: 9.5px;
  margin-right: 5px;
}

@media (max-width: 720px) {
  .algo pre { font-size: 11.5px; }
  .stage-canvas { padding: 10px; }
}

/* ───────── Table of contents (index page) ───────── */
.toc-hero {
  padding: 60px 0 36px;
  border-bottom: 1px solid var(--hairline);
}
.toc-hero .eyebrow { display: block; margin-bottom: 16px; }
.toc-hero h1 {
  font-family: var(--font-display);
  font-size: clamp(40px, 6vw, 72px);
  line-height: 1.02;
  letter-spacing: -0.025em;
}
.toc-hero .standfirst {
  margin-top: 20px;
  font-size: clamp(17px, 1.7vw, 21px);
  font-style: italic;
  color: var(--muted);
  max-width: 56ch;
  line-height: 1.55;
}
.part-band {
  display: flex;
  align-items: baseline;
  gap: 14px;
  margin: 40px 0 18px;
  padding-bottom: 8px;
  border-bottom: 1px solid var(--hairline);
}
.part-band .part-num {
  font-family: var(--font-mono);
  font-size: 11px;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--accent);
}
.part-band .part-label {
  font-family: var(--font-display);
  font-size: 17px;
  color: var(--ink);
}
.ch-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: 16px;
  margin: 26px 0 10px;
}
.ch-card {
  display: block;
  border: 1px solid var(--hairline);
  background: var(--cream);
  padding: 22px 22px 20px;
  border-radius: 3px;
  text-decoration: none;
  color: inherit;
  transition: border-color 0.18s ease, transform 0.18s ease, box-shadow 0.18s ease;
}
.ch-card:hover {
  border-color: var(--accent);
  transform: translateY(-2px);
  box-shadow: 0 8px 22px -14px var(--ink);
}
.ch-card .ch-num {
  display: inline-block;
  font-family: var(--font-mono);
  font-size: 11px;
  letter-spacing: 0.1em;
  color: var(--accent);
  margin-bottom: 10px;
}
.ch-card h2,
.ch-card h3 {
  font-family: var(--font-display);
  font-size: 21px;
  line-height: 1.2;
  letter-spacing: -0.01em;
  margin: 0 0 8px;
}
.ch-card p {
  font-size: 14px;
  line-height: 1.55;
  color: var(--muted);
  margin: 0 0 12px;
}
.ch-card .ch-tags {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
}
.ch-card .ch-tags span {
  font-family: var(--font-mono);
  font-size: 9.5px;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  color: var(--muted);
  border: 1px solid var(--hairline);
  padding: 3px 7px;
  border-radius: 2px;
}