/* ════════════════════════════════════════════════════════════════
   ebook.css — the case-study reader on the whitepaper design system.
   Cool paper · ink text · one deep signal-blue accent.
   Grotesk display + mono data. arXiv-meets-Stripe-docs.
   A re-skin of the paginated reader shell (formerly the dark
   "Determinism Layer" gold/serif skin) to match index.html.
   ════════════════════════════════════════════════════════════════ */

:root {
  /* ── Surface (cool paper, never pure white) ──────────────── */
  --paper:        #F4F5F6;
  --paper-raised: #FBFBFC;

  /* ── Hairlines ───────────────────────────────────────────── */
  --rule:         #E4E6E9;
  --rule-strong:  #CDD1D6;

  /* ── Ink (cool near-black, never pure black) ─────────────── */
  --ink:          #14161A;
  --ink-muted:    #565B62;
  --ink-faint:    #8A9099;

  /* ── Accent — deep signal blue, used with intent ─────────── */
  --accent:       #1B3BD8;
  --accent-deep:  #142C9E;
  --accent-soft:  rgba(27, 59, 216, 0.07);
  --accent-line:  rgba(27, 59, 216, 0.32);
  --on-accent:    #FFFFFF;

  /* ── Semantic (used sparingly, for real state) ───────────── */
  --ok:           #2E7D57;
  --ok-soft:      rgba(46, 125, 87, 0.08);
  --ok-line:      rgba(46, 125, 87, 0.28);
  --warn:         #B23B3B;
  --warn-soft:    rgba(178, 59, 59, 0.05);

  /* ── Type ────────────────────────────────────────────────── */
  --grotesk: "Space Grotesk", system-ui, sans-serif;  /* display + UI */
  --mono:    "IBM Plex Mono", ui-monospace, monospace; /* data / labels */

  --text-xs:   0.6875rem;
  --text-sm:   0.8125rem;
  --text-base: 1rem;
  --text-lg:   1.1875rem;
  --text-xl:   1.5rem;
  --text-2xl:  1.9375rem;

  --r: 2px;

  --header-h: 48px;
  --footer-h: 52px;
  --pad:  clamp(1.25rem, 5vw, 3rem);
}

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

html, body { height: 100%; overflow: hidden; }

body {
  font-family: var(--grotesk);
  font-size: var(--text-base);
  line-height: 1.6;
  background: var(--paper);
  color: var(--ink);
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

code { font-family: var(--mono); }

/* ── App header (light bar, hairline base, 48px) ────────────── */
.app-header {
  position: fixed;
  top: 0; left: 0; right: 0;
  height: var(--header-h);
  background: rgba(244, 245, 246, 0.82);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--rule);
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 var(--pad, 24px);
  z-index: 100;
}

.app-header-link {
  font-family: var(--mono);
  font-size: var(--text-xs);
  color: var(--ink-muted);
  text-decoration: none;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  transition: color 0.15s ease;
}
.app-header-link:hover { color: var(--ink); }
.app-header-link:focus-visible,
.ebook-footer-btn:focus-visible,
.rail-item:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
  border-radius: 1px;
}

/* Breadcrumb */
.app-breadcrumb { display: flex; align-items: center; gap: 8px; }
.app-breadcrumb-sep {
  font-family: var(--mono);
  font-size: var(--text-xs);
  color: var(--rule-strong);
}
.app-breadcrumb-current {
  font-family: var(--mono);
  font-size: var(--text-xs);
  color: var(--accent-deep);
  letter-spacing: 0.06em;
}

/* ── App shell (full-height area below header) ──────────────── */
.app-shell {
  position: fixed;
  top: var(--header-h);
  left: 0; right: 0; bottom: 0;
  display: flex;
}
.app-shell.has-footer { bottom: var(--footer-h); }

/* ── Left sidebar (chapter rail) ────────────────────────────── */
.app-sidebar {
  width: 200px;
  flex-shrink: 0;
  border-right: 1px solid var(--rule);
  background: var(--paper);
  overflow-y: auto;
  padding: 20px 0;
}

.sidebar-label {
  font-family: var(--mono);
  font-size: 0.625rem;
  font-weight: 500;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--ink-faint);
  padding: 0 20px;
  margin-bottom: 12px;
}

.rail-item {
  display: flex;
  align-items: baseline;
  gap: 10px;
  padding: 9px 20px;
  cursor: pointer;
  border-left: 2px solid transparent;
  transition: background 0.15s ease, color 0.15s ease;
}
.rail-item:hover .rail-item-title { color: var(--ink); }
.rail-item:hover .rail-item-num { color: var(--ink-muted); }
.rail-item.active {
  border-left-color: var(--accent);
  background: var(--accent-soft);
}

.rail-item-num {
  font-family: var(--mono);
  font-size: var(--text-xs);
  color: var(--ink-faint);
  font-weight: 500;
  min-width: 22px;
  flex-shrink: 0;
  text-align: right;
}
.rail-item.active .rail-item-num { color: var(--accent-deep); }

.rail-item-title {
  font-family: var(--grotesk);
  font-size: var(--text-sm);
  color: var(--ink-muted);
  line-height: 1.35;
  transition: color 0.15s ease;
}
.rail-item.active .rail-item-title { color: var(--ink); font-weight: 500; }

/* ── Right content area ─────────────────────────────────────── */
.app-content {
  flex: 1;
  overflow-y: auto;
  min-width: 0;
  position: relative;
  background: var(--paper);
}

/* ── Chapter panels ─────────────────────────────────────────── */
.chapter { display: none; min-height: 100%; }
.chapter.active { display: block; }

.chapter.interior {
  color: var(--ink);
  /* faint engineering-paper grid, as a watermark */
  background-color: var(--paper);
  background-image:
    linear-gradient(rgba(205, 209, 214, 0.35) 1px, transparent 1px),
    linear-gradient(90deg, rgba(205, 209, 214, 0.35) 1px, transparent 1px);
  background-size: 44px 44px;
  background-position: -1px -1px;
}

/* ── Page-turn animations ───────────────────────────────────── */
@keyframes exitLeft  { from { opacity: 1; transform: translateX(0); }   to { opacity: 0; transform: translateX(-36px); } }
@keyframes exitRight { from { opacity: 1; transform: translateX(0); }   to { opacity: 0; transform: translateX(36px);  } }
@keyframes enterFromRight { from { opacity: 0; transform: translateX(36px);  } to { opacity: 1; transform: translateX(0); } }
@keyframes enterFromLeft  { from { opacity: 0; transform: translateX(-36px); } to { opacity: 1; transform: translateX(0); } }

.chapter.exit-left,
.chapter.exit-right {
  display: block !important;
  position: absolute;
  top: 0; left: 0; right: 0;
  pointer-events: none;
}
.chapter.exit-left  { animation: exitLeft 0.22s ease forwards; }
.chapter.exit-right { animation: exitRight 0.22s ease forwards; }
.chapter.enter-from-right { animation: enterFromRight 0.26s ease forwards; }
.chapter.enter-from-left  { animation: enterFromLeft 0.26s ease forwards; }

/* ════════════════════════════════════════════════════════════
   Interior chapter layout
   ════════════════════════════════════════════════════════════ */
.ch-inner {
  max-width: 720px;
  padding: clamp(2.5rem, 6vw, 3.5rem) clamp(1.5rem, 5vw, 3rem) 5rem;
}

.ch-header {
  margin-bottom: 3rem;
  padding-bottom: 1.5rem;
  border-bottom: 2px solid var(--ink);
}

.ch-eyebrow {
  font-family: var(--mono);
  font-size: var(--text-xs);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--accent-deep);
  font-weight: 500;
  margin-bottom: 0.9rem;
}

.ch-title {
  font-family: var(--grotesk);
  font-weight: 500;
  font-size: clamp(1.9rem, 4.4vw, 2.9rem);
  line-height: 1.1;
  letter-spacing: -0.025em;
  color: var(--ink);
}

.ch-subhead {
  font-family: var(--grotesk);
  font-weight: 500;
  font-size: var(--text-xl);
  letter-spacing: -0.02em;
  color: var(--ink);
  margin: 2.75rem 0 0.25rem;
}

/* Body prose */
.ch-body {
  font-family: var(--grotesk);
  font-size: 1.0625rem;
  line-height: 1.72;
  color: var(--ink-muted);
}
.ch-body p + p { margin-top: 1.4rem; }
.ch-body em { color: var(--ink); font-style: italic; }
.ch-body strong { color: var(--ink); font-weight: 500; }

/* Drop cap */
.drop-cap::first-letter {
  font-family: var(--grotesk);
  font-weight: 500;
  font-size: 3.9em;
  line-height: 0.76;
  float: left;
  margin: 0.05em 0.12em 0 0;
  color: var(--accent-deep);
}

/* Inline code in prose */
.ch-body code,
.tech-list code,
.principle-body code,
.pipeline-stage-desc code,
.limitation-detail code {
  font-family: var(--mono);
  font-size: 0.82em;
  background: var(--accent-soft);
  color: var(--accent-deep);
  padding: 1px 5px;
  border-radius: var(--r);
}

/* ── Pull quote ─────────────────────────────────────────────── */
.pull-quote {
  margin: 2.75rem 0;
  padding: 0.25rem 0 0.25rem 1.75rem;
  border-left: 2px solid var(--accent);
}
.pull-quote p {
  font-family: var(--grotesk);
  font-style: italic;
  font-weight: 400;
  font-size: var(--text-xl);
  line-height: 1.42;
  color: var(--ink);
  letter-spacing: -0.01em;
}

/* ── Aside block ────────────────────────────────────────────── */
.aside {
  margin: 2.5rem 0;
  padding: 1.25rem 1.5rem;
  border-left: 2px solid var(--accent);
  background: var(--accent-soft);
  border-radius: 0 var(--r) var(--r) 0;
}
.aside-label {
  font-family: var(--mono);
  font-size: var(--text-xs);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--accent-deep);
  font-weight: 500;
  margin-bottom: 0.6rem;
}
.aside p {
  font-family: var(--grotesk);
  font-size: var(--text-sm);
  line-height: 1.7;
  color: var(--ink-muted);
}
.aside p code {
  font-family: var(--mono);
  font-size: 0.85em;
  background: rgba(27, 59, 216, 0.1);
  color: var(--accent-deep);
  padding: 1px 4px;
  border-radius: var(--r);
}

/* ── Stat row ───────────────────────────────────────────────── */
.stat-row {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  border: 1px solid var(--rule);
  border-radius: var(--r);
  overflow: hidden;
  margin: 2.75rem 0;
  background: var(--paper-raised);
}
.stat-row-cell {
  padding: 1.25rem 1.25rem 1rem;
  border-right: 1px solid var(--rule);
}
.stat-row-cell:last-child { border-right: none; }
.stat-row-num {
  font-family: var(--grotesk);
  font-weight: 500;
  font-size: 1.75rem;
  color: var(--accent-deep);
  line-height: 1;
  letter-spacing: -0.03em;
  margin-bottom: 0.4rem;
}
.stat-row-label {
  font-family: var(--mono);
  font-size: 0.625rem;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--ink-faint);
  line-height: 1.4;
}

/* ── Agent section (Ch III/IV) ──────────────────────────────── */
.agent-section {
  margin-top: 3rem;
  padding-top: 2.5rem;
  border-top: 1px solid var(--rule);
}
.agent-section:first-of-type { margin-top: 2.25rem; }

.agent-header {
  display: flex;
  align-items: baseline;
  gap: 1rem;
  margin-bottom: 1.25rem;
}
.agent-name {
  font-family: var(--grotesk);
  font-weight: 500;
  font-size: var(--text-2xl);
  letter-spacing: -0.02em;
  color: var(--ink);
  line-height: 1;
}
.agent-tag {
  font-family: var(--mono);
  font-size: 0.625rem;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--accent-deep);
  padding: 3px 8px;
  border: 1px solid var(--accent-line);
  border-radius: var(--r);
}

/* Technical detail list */
.tech-list {
  list-style: none;
  margin-top: 1.5rem;
  display: flex;
  flex-direction: column;
  gap: 0.65rem;
}
.tech-list li {
  font-family: var(--grotesk);
  font-size: var(--text-sm);
  line-height: 1.6;
  color: var(--ink-muted);
  padding-left: 1.25rem;
  position: relative;
}
.tech-list li::before {
  content: "→";
  position: absolute;
  left: 0;
  color: var(--accent-deep);
  font-family: var(--mono);
  font-size: var(--text-xs);
}

/* ── Outcome section ────────────────────────────────────────── */
.outcome-section {
  margin-top: 3.25rem;
  padding-top: 2.25rem;
  border-top: 2px solid var(--ink);
}
.outcome-label {
  font-family: var(--mono);
  font-size: var(--text-xs);
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--ink-faint);
  margin-bottom: 1rem;
}
.outcome-metrics {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  border: 1px solid var(--rule);
  border-radius: var(--r);
  overflow: hidden;
  margin: 1.75rem 0 2.25rem;
  background: var(--paper-raised);
}
.outcome-metric {
  padding: 1.25rem 1.4rem;
  border-right: 1px solid var(--rule);
}
.outcome-metric:last-child { border-right: none; }
.outcome-metric-val {
  font-family: var(--grotesk);
  font-weight: 500;
  font-size: 1.85rem;
  color: var(--accent-deep);
  line-height: 1;
  letter-spacing: -0.03em;
  margin-bottom: 0.4rem;
}
.outcome-metric-label {
  font-family: var(--mono);
  font-size: 0.625rem;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--ink-faint);
  line-height: 1.5;
}

/* ── Code block (dark panel, Stripe-docs style) ─────────────── */
.code-block {
  background: #131417;
  border: 1px solid #262A31;
  border-radius: var(--r);
  margin: 2.25rem 0;
  overflow: hidden;
}
.code-block-label {
  display: block;
  font-family: var(--mono);
  font-size: 0.625rem;
  letter-spacing: 0.06em;
  color: #7C828C;
  background: #1B1D22;
  border-bottom: 1px solid #262A31;
  padding: 7px 14px;
  text-transform: uppercase;
}
.code-block pre {
  font-family: var(--mono);
  font-size: 0.78rem;
  line-height: 1.75;
  color: #C7CAD1;
  margin: 0;
  white-space: pre;
  padding: 1.1rem 1.5rem;
  overflow-x: auto;
}
.code-block .cm { color: #6B7078; }       /* comment */
.code-block .ck { color: #8AA6FF; }       /* keyword */
.code-block .cs { color: #8FBBA0; }       /* string  */

/* Bare inline code block (no label chrome) */
.ch-code {
  font-family: var(--mono);
  font-size: 0.78rem;
  line-height: 1.75;
  color: #C7CAD1;
  background: #131417;
  border: 1px solid #262A31;
  border-radius: var(--r);
  margin: 2.25rem 0;
  padding: 1.1rem 1.5rem;
  overflow-x: auto;
  white-space: pre;
}
.ch-code code { font-family: var(--mono); color: inherit; background: none; padding: 0; }

/* ── Tables (arch + capability) ─────────────────────────────── */
.arch-table,
.cap-table {
  width: 100%;
  border-collapse: collapse;
  margin: 2.25rem 0;
  border: 1px solid var(--rule);
  border-radius: var(--r);
  overflow: hidden;
  background: var(--paper-raised);
}
.arch-table th,
.cap-table th {
  font-family: var(--mono);
  font-size: 0.625rem;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--ink-faint);
  padding: 12px 18px;
  text-align: left;
  border-bottom: 1px solid var(--rule);
  background: var(--paper);
}
.arch-table td,
.cap-table td {
  font-family: var(--grotesk);
  font-size: var(--text-sm);
  line-height: 1.55;
  padding: 13px 18px;
  border-bottom: 1px solid var(--rule);
  vertical-align: top;
  color: var(--ink);
}
.arch-table tr:last-child td,
.cap-table tr:last-child td { border-bottom: none; }

.arch-table td:first-child,
.cap-table td:first-child {
  font-family: var(--mono);
  font-size: var(--text-xs);
  color: var(--accent-deep);
  white-space: nowrap;
}
.arch-table td:nth-child(2) {
  font-family: var(--mono);
  font-size: var(--text-xs);
  color: var(--ink-muted);
  white-space: nowrap;
}

/* Capability badges (semantic role tags) */
.cap-badge {
  font-family: var(--mono);
  font-size: 0.625rem;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  padding: 2px 7px;
  border-radius: var(--r);
  white-space: nowrap;
}
.cap-badge.customer {
  color: var(--ok);
  background: var(--ok-soft);
  border: 1px solid var(--ok-line);
}
.cap-badge.admin {
  color: var(--accent-deep);
  background: var(--accent-soft);
  border: 1px solid var(--accent-line);
}

/* ── Permission grid (allow / block) ────────────────────────── */
.permission-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 1px;
  background: var(--rule);
  border: 1px solid var(--rule);
  border-radius: var(--r);
  overflow: hidden;
  margin: 2rem 0;
}
.permission-col { background: var(--paper-raised); padding: 1.25rem 1.4rem; }
.permission-col-label {
  font-family: var(--mono);
  font-size: 0.625rem;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  margin-bottom: 0.9rem;
  font-weight: 500;
}
.permission-col-label.allow { color: var(--ok); }
.permission-col-label.block { color: var(--warn); }

.permission-list { list-style: none; display: flex; flex-direction: column; gap: 0.5rem; }
.permission-list li {
  font-family: var(--mono);
  font-size: var(--text-xs);
  line-height: 1.45;
  padding-left: 1.1rem;
  position: relative;
  color: var(--ink-muted);
}
.allow .permission-list li::before {
  content: "✓"; position: absolute; left: 0; color: var(--ok); font-size: 0.625rem;
}
.block .permission-list li::before {
  content: "✕"; position: absolute; left: 0; color: var(--warn); font-size: 0.625rem;
}

/* ── Timeline (Ch VI) ───────────────────────────────────────── */
.timeline { display: flex; flex-direction: column; margin-top: 0.5rem; }
.timeline-item {
  display: grid;
  grid-template-columns: 130px 1fr;
  gap: 2.25rem;
  padding: 2rem 0;
  border-bottom: 1px solid var(--rule);
}
.timeline-item:last-child { border-bottom: none; }
.timeline-period {
  font-family: var(--mono);
  font-size: var(--text-xs);
  color: var(--ink-faint);
  line-height: 1.6;
  padding-top: 3px;
}
.timeline-role {
  font-family: var(--grotesk);
  font-size: var(--text-base);
  font-weight: 500;
  color: var(--ink);
  margin-bottom: 3px;
}
.timeline-company {
  font-family: var(--mono);
  font-size: var(--text-xs);
  color: var(--accent-deep);
  letter-spacing: 0.04em;
  margin-bottom: 0.9rem;
}
.timeline-bullets { list-style: none; display: flex; flex-direction: column; gap: 0.5rem; }
.timeline-bullets li {
  font-family: var(--grotesk);
  font-size: var(--text-sm);
  line-height: 1.6;
  color: var(--ink-muted);
  padding-left: 1.1rem;
  position: relative;
}
.timeline-bullets li::before {
  content: "-"; position: absolute; left: 0; color: var(--ink-faint); font-family: var(--mono);
}
.timeline-note {
  font-family: var(--grotesk);
  font-style: italic;
  font-size: var(--text-sm);
  color: var(--ink-faint);
  margin-top: 0.65rem;
}

/* ── Layer heading ──────────────────────────────────────────── */
.layer-heading {
  font-family: var(--grotesk);
  font-weight: 500;
  font-size: var(--text-xl);
  letter-spacing: -0.02em;
  color: var(--ink);
  margin: 2.75rem 0 1rem;
  padding-top: 2.25rem;
  border-top: 1px solid var(--rule);
}
.layer-num {
  font-family: var(--mono);
  font-size: 0.625rem;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--accent-deep);
  font-weight: 500;
  display: block;
  margin-bottom: 0.4rem;
}

/* ── Pipeline stages (Ch V) ─────────────────────────────────── */
.pipeline-stages {
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: 1px;
  background: var(--rule);
  border: 1px solid var(--rule);
  border-radius: var(--r);
  overflow: hidden;
  margin: 2.25rem 0;
}
.pipeline-stage { background: var(--paper-raised); padding: 1.25rem 1.25rem 1.5rem; }
.pipeline-stage-num {
  font-family: var(--mono);
  font-size: 0.625rem;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--ink-faint);
  margin-bottom: 0.5rem;
}
.pipeline-stage-name {
  font-family: var(--grotesk);
  font-weight: 500;
  font-size: var(--text-lg);
  letter-spacing: -0.01em;
  color: var(--ink);
  line-height: 1;
  margin-bottom: 0.4rem;
}
.pipeline-stage-budget {
  font-family: var(--mono);
  font-size: var(--text-xs);
  color: var(--accent-deep);
  margin-bottom: 0.9rem;
}
.pipeline-stage-desc {
  font-family: var(--grotesk);
  font-size: var(--text-sm);
  line-height: 1.6;
  color: var(--ink-muted);
  padding-top: 0.75rem;
  border-top: 1px solid var(--rule);
}

/* ── Security callout (semantic warning) ────────────────────── */
.security-callout {
  margin: 2rem 0;
  padding: 1.25rem 1.5rem;
  border-left: 2px solid var(--warn);
  background: var(--warn-soft);
  border-radius: 0 var(--r) var(--r) 0;
}
.security-callout-label {
  font-family: var(--mono);
  font-size: var(--text-xs);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--warn);
  font-weight: 500;
  margin-bottom: 0.6rem;
}
.security-callout p {
  font-family: var(--grotesk);
  font-size: var(--text-sm);
  line-height: 1.7;
  color: var(--ink-muted);
  margin: 0;
}

/* ── Principle cards (Ch V) ─────────────────────────────────── */
.principle-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 1px;
  background: var(--rule);
  border: 1px solid var(--rule);
  border-radius: var(--r);
  overflow: hidden;
  margin: 2rem 0 0.75rem;
}
.principle-card { background: var(--paper-raised); padding: 1.4rem 1.5rem; display: flex; flex-direction: column; }
.principle-card.full { grid-column: 1 / -1; }
.principle-num {
  font-family: var(--mono);
  font-size: 0.625rem;
  letter-spacing: 0.14em;
  color: var(--accent-deep);
  margin-bottom: 0.65rem;
}
.principle-title {
  font-family: var(--grotesk);
  font-weight: 500;
  font-size: var(--text-lg);
  line-height: 1.2;
  letter-spacing: -0.01em;
  color: var(--ink);
  margin: 0 0 0.65rem;
}
.principle-body {
  font-family: var(--grotesk);
  font-size: var(--text-sm);
  line-height: 1.6;
  color: var(--ink-muted);
  margin: 0;
}
.principle-foil {
  margin-top: 0.9rem;
  padding-top: 0.75rem;
  border-top: 1px dashed var(--rule-strong);
}
.principle-foil-label {
  font-family: var(--mono);
  font-size: 0.5625rem;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--ink-faint);
  display: block;
  margin-bottom: 0.4rem;
}
.principle-foil p {
  font-family: var(--grotesk);
  font-style: italic;
  font-size: var(--text-sm);
  line-height: 1.55;
  color: var(--ink-faint);
  margin: 0;
}

/* ── Limitations ledger (Ch V) ──────────────────────────────── */
.limitation-ledger {
  margin: 1.75rem 0 0.75rem;
  border: 1px solid var(--rule);
  border-radius: var(--r);
  overflow: hidden;
  background: var(--paper-raised);
}
.limitation-row {
  display: grid;
  grid-template-columns: 230px 1fr;
  border-bottom: 1px solid var(--rule);
}
.limitation-row:last-child { border-bottom: none; }
.limitation-name {
  padding: 1.1rem 1.25rem;
  background: var(--accent-soft);
  border-right: 1px solid var(--rule);
  font-family: var(--grotesk);
  font-weight: 500;
  font-size: var(--text-sm);
  line-height: 1.35;
  color: var(--ink);
}
.limitation-glyph {
  display: block;
  font-family: var(--mono);
  font-size: var(--text-xs);
  color: var(--accent-deep);
  letter-spacing: 0.06em;
  margin-bottom: 0.4rem;
}
.limitation-detail { padding: 1.1rem 1.4rem; }
.limitation-detail p {
  font-family: var(--grotesk);
  font-size: var(--text-sm);
  line-height: 1.7;
  color: var(--ink-muted);
  margin: 0;
}
.limitation-direction {
  margin-top: 0.75rem;
  padding-top: 0.65rem;
  border-top: 1px dashed var(--rule-strong);
  font-family: var(--mono);
  font-size: var(--text-xs);
  line-height: 1.55;
  color: var(--ink-muted);
}
.limitation-direction strong {
  font-weight: 500;
  color: var(--accent-deep);
  letter-spacing: 0.08em;
  text-transform: uppercase;
  margin-right: 0.65rem;
  font-size: 0.625rem;
}

/* ── OS diagrams (Ch II) ────────────────────────────────────── */
.os-diagram-wrap,
.os-lifecycle-wrap {
  margin: 2.75rem 0;
  border: 1px solid var(--rule);
  border-radius: var(--r);
  overflow: hidden;
  background: var(--paper-raised);
}
.os-diagram-wrap svg,
.os-lifecycle-wrap svg { width: 100%; height: auto; display: block; }

.os-node { opacity: 0; }
.os-arrow { stroke-dasharray: 300; stroke-dashoffset: 300; }
.os-config-band { opacity: 0; }

.os-animate .os-node { animation: osNodeIn 0.5s ease forwards; }
.os-animate .os-arrow { animation: osArrowDraw 0.45s ease forwards; }
.os-animate .os-config-band { animation: osFadeIn 0.55s ease forwards; }

.os-animate .os-node:nth-child(1)  { animation-delay: 0.05s; }
.os-animate .os-node:nth-child(2)  { animation-delay: 0.15s; }
.os-animate .os-node:nth-child(3)  { animation-delay: 0.25s; }
.os-animate .os-node:nth-child(4)  { animation-delay: 0.35s; }
.os-animate .os-node:nth-child(5)  { animation-delay: 0.45s; }
.os-animate .os-node:nth-child(6)  { animation-delay: 0.55s; }
.os-animate .os-arrow:nth-child(1) { animation-delay: 0.3s; }
.os-animate .os-arrow:nth-child(2) { animation-delay: 0.4s; }
.os-animate .os-arrow:nth-child(3) { animation-delay: 0.5s; }
.os-animate .os-arrow:nth-child(4) { animation-delay: 0.6s; }
.os-animate .os-arrow:nth-child(5) { animation-delay: 0.7s; }
.os-config-band { animation-delay: 0.75s; }

@keyframes osNodeIn { from { opacity: 0; transform: translateY(8px); } to { opacity: 1; transform: translateY(0); } }
@keyframes osArrowDraw { to { stroke-dashoffset: 0; } }
@keyframes osFadeIn { from { opacity: 0; } to { opacity: 1; } }

.os-node-group { cursor: default; }
.os-node-group:hover .os-node-rect { filter: brightness(0.985); }

/* ── Figures / captions ─────────────────────────────────────── */
.ch-body figure { margin: 1.75rem 0 0.5rem; }
.ch-body figure img { border-radius: var(--r); }
.ch-body figcaption {
  font-family: var(--mono);
  font-size: var(--text-xs);
  color: var(--ink-faint);
  margin-top: 0.5rem;
  letter-spacing: 0.02em;
  line-height: 1.5;
}
.ch-body figcaption code {
  font-family: var(--mono);
  font-size: 0.9em;
  color: var(--accent-deep);
}

/* ── Ebook footer (light bar, hairline top, 52px) ───────────── */
.ebook-footer {
  position: fixed;
  bottom: 0; left: 0; right: 0;
  height: var(--footer-h);
  background: rgba(244, 245, 246, 0.86);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  border-top: 1px solid var(--rule);
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 var(--pad, 24px);
  z-index: 100;
}
.ebook-footer-btn {
  font-family: var(--mono);
  font-size: var(--text-xs);
  letter-spacing: 0.06em;
  color: var(--ink-muted);
  background: none;
  border: none;
  cursor: pointer;
  padding: 8px 0;
  min-width: 120px;
  transition: color 0.15s ease;
}
.ebook-footer-btn:hover { color: var(--accent-deep); }
.ebook-footer-btn:disabled { opacity: 0; pointer-events: none; }
#footer-prev { text-align: left; }
#footer-next { text-align: right; }
.ebook-footer-indicator {
  font-family: var(--mono);
  font-size: 0.625rem;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--ink-faint);
  text-align: center;
  flex: 1;
  padding: 0 16px;
}

/* ════════════════════════════════════════════════════════════
   Mobile
   ════════════════════════════════════════════════════════════ */
@media (max-width: 768px) {
  .app-header { padding: 0 14px; }
  .app-breadcrumb .app-header-link,
  .app-breadcrumb .app-breadcrumb-sep:first-child { display: none; }
  .app-breadcrumb-current {
    font-size: 0.625rem;
    max-width: 60vw;
    overflow: hidden;
    text-overflow: ellipsis;
    white-space: nowrap;
  }

  /* Sidebar becomes a horizontal chapter chip-rail above content */
  .app-shell { flex-direction: column; }
  .app-sidebar {
    width: 100%;
    height: auto;
    flex-shrink: 0;
    border-right: none;
    border-bottom: 1px solid var(--rule);
    overflow-x: auto;
    overflow-y: hidden;
    padding: 8px 12px;
    display: flex;
    align-items: stretch;
    gap: 4px;
    scrollbar-width: none;
  }
  .app-sidebar::-webkit-scrollbar { display: none; }
  .sidebar-label { display: none; }
  .rail-item {
    flex-shrink: 0;
    padding: 6px 10px;
    border-left: none;
    border-bottom: 2px solid transparent;
    gap: 6px;
  }
  .rail-item.active { border-left-color: transparent; border-bottom-color: var(--accent); }
  .rail-item-title { white-space: nowrap; }

  /* Chapter layout */
  .ch-inner { max-width: 100%; padding: 1.5rem 1.125rem 3.5rem; }
  .ch-header { margin-bottom: 2rem; padding-bottom: 1.25rem; }
  .ch-title { font-size: clamp(1.7rem, 8vw, 2rem); line-height: 1.1; }
  .ch-body { font-size: var(--text-base); line-height: 1.66; }
  .ch-body p + p { margin-top: 1.15rem; }
  .drop-cap::first-letter { font-size: 3.2em; line-height: 0.8; }

  .pull-quote { margin: 2rem 0; padding-left: 1.15rem; }
  .pull-quote p { font-size: var(--text-lg); line-height: 1.45; }

  .aside { margin: 1.75rem 0; padding: 1rem 1.15rem; }

  /* Stat row: 4-col → 2x2 */
  .stat-row { grid-template-columns: repeat(2, 1fr); margin: 2rem 0; }
  .stat-row-cell {
    padding: 1rem 0.9rem;
    border-right: 1px solid var(--rule);
    border-bottom: 1px solid var(--rule);
  }
  .stat-row-cell:nth-child(2n) { border-right: none; }
  .stat-row-cell:nth-last-child(-n+2) { border-bottom: none; }
  .stat-row-num { font-size: 1.4rem; }

  /* Agent */
  .agent-header { flex-wrap: wrap; gap: 0.65rem; }
  .agent-name { font-size: var(--text-xl); }

  /* Tables */
  .cap-table, .arch-table { margin: 1.75rem 0; }
  .cap-table td, .arch-table td { padding: 11px 12px; font-size: var(--text-xs); }
  .cap-table th, .arch-table th { padding: 10px 12px; }
  .cap-table td:first-child,
  .arch-table td:first-child,
  .arch-table td:nth-child(2) { white-space: normal; word-break: break-word; }

  /* Code block */
  .code-block { margin: 1.75rem 0; }
  .code-block pre { font-size: 0.72rem; padding: 0.9rem 1rem; }

  /* Grids → single column */
  .pipeline-stages { grid-template-columns: 1fr; margin: 1.75rem 0; }
  .pipeline-stage { padding: 1rem 1.15rem; }
  .outcome-metrics { grid-template-columns: 1fr; }
  .outcome-metric { padding: 1rem 1.15rem; border-right: none; border-bottom: 1px solid var(--rule); }
  .outcome-metric:last-child { border-bottom: none; }
  .outcome-metric-val { font-size: 1.5rem; }
  .principle-grid { grid-template-columns: 1fr; margin: 1.5rem 0; }
  .principle-card { padding: 1.15rem 1.25rem; }
  .permission-grid { grid-template-columns: 1fr; margin: 1.5rem 0; }
  .permission-col { padding: 1rem 1.15rem; }

  /* Timeline */
  .timeline-item { grid-template-columns: 1fr; gap: 0.5rem; padding: 1.4rem 0; }
  .timeline-period { padding-top: 0; }
  .timeline-company { margin-bottom: 0.65rem; }

  /* Limitation ledger */
  .limitation-row { grid-template-columns: 1fr; }
  .limitation-name { border-right: none; border-bottom: 1px solid var(--rule); padding: 0.9rem 1rem; }
  .limitation-detail { padding: 0.9rem 1rem; }

  .layer-heading { font-size: var(--text-lg); margin: 2.25rem 0 0.9rem; padding-top: 1.75rem; }
  .ch-subhead { font-size: var(--text-lg); }
  .outcome-section { margin-top: 2.25rem; padding-top: 1.5rem; }
  .agent-section { margin-top: 2.25rem; padding-top: 1.75rem; }

  /* Footer */
  .ebook-footer { padding: 0 14px; }
  .ebook-footer-btn { min-width: 0; padding: 8px 6px; font-size: 0.625rem; }
  .ebook-footer-indicator { display: none; }
}

@media (max-width: 480px) {
  .app-header { padding: 0 10px; }
  .app-header-link { font-size: 0.5625rem; }
  .app-breadcrumb-current { font-size: 0.5625rem; max-width: 50vw; }
  .ch-inner { padding: 1.25rem 0.875rem 3rem; }
  .ch-title { font-size: 1.65rem; line-height: 1.12; }
  .ch-body { font-size: 0.9375rem; }
  .drop-cap::first-letter { font-size: 2.9em; }
  .pull-quote p { font-size: var(--text-base); }
  .stat-row-num { font-size: 1.25rem; }
  .agent-name { font-size: var(--text-lg); }
  .pipeline-stage-name { font-size: var(--text-base); }
  .code-block pre { font-size: 0.7rem; padding: 0.8rem 0.9rem; }
}

/* ── Reduced motion ─────────────────────────────────────────── */
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { animation-duration: 0.01ms !important; transition-duration: 0.01ms !important; }
  .os-node, .os-config-band { opacity: 1 !important; }
  .os-arrow { stroke-dashoffset: 0 !important; }
}