:root {
  --color-surface: #ffffff;
  --color-surface-alt: #f6f6f7;
  --color-surface-deep: #e9e9eb;
  --color-ink: #484b56;
  --color-ink-soft: #6d707e;
  --color-ink-muted: #777b88;
  --color-line: #dedfe1;
  --color-primary: #4d61b2;
  --color-primary-ink: #ffffff;
  --color-primary-soft: #e6e9f4;
  --color-accent: #c64439;
  --color-accent-ink: #ffffff;
  --color-highlight: #f5dddb;
  --color-focus: #d3392c;
  --color-contrast-surface: #1b1e27;
  --color-contrast-ink: #e4e5e7;
  --color-contrast-line: #474a51;
  --color-contrast-accent: #d0665d;
  --overlay-scrim: linear-gradient(180deg, rgba(27, 30, 39, 0.35) 0%, rgba(27, 30, 39, 0.72) 55%, rgba(27, 30, 39, 0.92) 100%);
  --font-heading: "Helvetica Neue", Arial, "PingFang SC", "Microsoft YaHei", sans-serif;
  --font-body: "Helvetica Neue", Arial, "PingFang SC", "Microsoft YaHei", sans-serif;
  --font-mono: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  --text-xs: 12.2px;
  --text-sm: 14.6px;
  --text-base: 16px;
  --text-lg: 19.2px;
  --text-xl: 23.0px;
  --text-2xl: 27.6px;
  --text-3xl: 33.2px;
  --text-4xl: 39.8px;
  --text-display: clamp(34px, 5.3vw, 68px);
  --text-figure: clamp(28px, 3.8vw, 49px);
  --weight-heading: 673;
  --weight-body: 400;
  --leading-body: 1.69;
  --leading-heading: 1.17;
  --tracking-heading: -0.006em;
  --radius-sm: 2px;
  --radius-md: 4px;
  --radius-lg: 6px;
  --radius-pill: 6px;
  --border-thin: 1px;
  --border-thick: 1px;
  --shadow-1: 0 1px 0 rgba(72, 75, 86, 0.12);
  --shadow-2: 0 2px 0 rgba(72, 75, 86, 0.18);
  --space-1: 8px;
  --space-2: 16px;
  --space-3: 24px;
  --space-4: 32px;
  --space-6: 48px;
  --space-8: 64px;
  --container: 1280px;
  --measure: 67ch;
  --gutter: 24px;
  --section-y: 63px;
  --section-y-lg: 88px;
}

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

html { -webkit-text-size-adjust: 100%; scroll-behavior: smooth; }

body,
h1, h2, h3, h4, h5, h6,
p, ul, ol, li, dl, dt, dd, figure, blockquote, pre {
  margin: 0;
  padding: 0;
}

ul, ol { list-style: none; }

img, svg, video { display: block; max-width: 100%; }

a { color: inherit; text-decoration: none; }

button,
input,
select,
textarea { font: inherit; color: inherit; }

button {
  background: none;
  border: 0;
  padding: 0;
  cursor: pointer;
}

table { border-collapse: collapse; width: 100%; }

hr { border: 0; }

/* ============ base typography ============ */
body {
  font-family: var(--font-body);
  font-size: var(--text-base);
  font-weight: var(--weight-body);
  line-height: var(--leading-body);
  color: var(--color-ink);
  background-color: var(--color-surface);
  letter-spacing: 0.01em;
  -webkit-font-smoothing: antialiased;
}

h1, h2, h3, h4, h5, h6 {
  font-family: var(--font-heading);
  font-weight: var(--weight-heading);
  line-height: var(--leading-heading);
  letter-spacing: var(--tracking-heading);
  color: var(--color-ink);
}

h1 { font-size: var(--text-3xl); letter-spacing: 0.06em; }
h2 { font-size: var(--text-2xl); letter-spacing: 0.07em; }
h3 { font-size: var(--text-xl); letter-spacing: 0.08em; }
h4 { font-size: var(--text-lg); letter-spacing: 0.09em; }

strong, b { font-weight: var(--weight-heading); }

.mono,
.idx,
time,
code,
kbd,
samp {
  font-family: var(--font-mono);
  font-variant-numeric: tabular-nums;
  letter-spacing: 0;
}

.idx {
  display: inline-flex;
  align-items: center;
  font-size: var(--text-xs);
  color: var(--color-ink-muted);
}

p + p { margin-top: var(--space-2); }

::selection {
  background-color: var(--color-primary-soft);
  color: var(--color-ink);
}

/* ============ focus ============ */
:focus-visible {
  outline: 2px solid var(--color-focus);
  outline-offset: 2px;
  border-radius: var(--radius-sm);
}

.skip-link {
  position: absolute;
  top: -140px;
  left: var(--space-2);
  z-index: 300;
  padding: 10px 18px;
  background-color: var(--color-primary);
  color: var(--color-primary-ink);
  border-radius: var(--radius-sm);
  font-size: var(--text-sm);
  letter-spacing: 0.06em;
  transition: top 0.16s ease;
}

.skip-link:focus {
  top: var(--space-2);
}

/* ============ layout ============ */
.container {
  width: 100%;
  max-width: var(--container);
  margin-inline: auto;
  padding-inline: var(--gutter);
}

.section {
  padding-block: var(--section-y);
}

.section--lg {
  padding-block: var(--section-y-lg);
}

.section--tight {
  padding-block: var(--space-4);
}

.grid {
  display: grid;
  grid-template-columns: repeat(12, minmax(0, 1fr));
  gap: var(--space-3);
}

.col-3 { grid-column: span 3; }
.col-4 { grid-column: span 4; }
.col-5 { grid-column: span 5; }
.col-6 { grid-column: span 6; }
.col-7 { grid-column: span 7; }
.col-8 { grid-column: span 8; }
.col-9 { grid-column: span 9; }
.col-12 { grid-column: span 12; }

.prose {
  max-width: var(--measure);
}

.prose p + p,
.prose ul + p,
.prose p + ul { margin-top: var(--space-2); }

.prose ul {
  display: grid;
  gap: var(--space-1);
}

.prose li {
  position: relative;
  padding-left: 18px;
}

.prose li::before {
  content: "";
  position: absolute;
  left: 0;
  top: 0.72em;
  width: 8px;
  height: 1px;
  background-color: var(--color-primary);
}

/* ============ bands ============ */
.band-deep {
  background-color: var(--color-contrast-surface);
  color: var(--color-contrast-ink);
}

.band-deep h1,
.band-deep h2,
.band-deep h3,
.band-deep h4 {
  color: var(--color-contrast-ink);
}

.band-alt {
  background-color: var(--color-surface-alt);
}

.band-deep .rule-dotted,
.band-deep .table th,
.band-deep .table td {
  border-color: var(--color-contrast-line);
}

.band-deep .table thead th {
  background-color: rgba(71, 74, 81, 0.45);
  color: var(--color-contrast-ink);
}

/* ============ rules & dividers ============ */
.rule-dotted {
  border-top: var(--border-thin) dashed var(--color-line);
}

.rule-solid {
  border-top: var(--border-thin) solid var(--color-line);
}

.rule-accent {
  border-top: 3px solid var(--color-accent);
}

.rule-open {
  height: 2px;
  border: 0;
  background-image: repeating-linear-gradient(
    to right,
    var(--color-line) 0 6px,
    transparent 6px 12px
  );
}

/* ============ chapter ============ */
.chapter {
  position: relative;
}

.chapter__head {
  display: flex;
  flex-wrap: wrap;
  align-items: baseline;
  gap: var(--space-2);
  padding-bottom: var(--space-2);
  border-bottom: var(--border-thin) dashed var(--color-line);
  margin-bottom: var(--space-3);
}

.chapter__num {
  font-family: var(--font-mono);
  font-size: var(--text-sm);
  color: var(--color-primary-ink);
  background-color: var(--color-primary);
  padding: 3px 8px;
  border-radius: var(--radius-sm);
  letter-spacing: 0.04em;
}

.chapter__title {
  font-size: var(--text-xl);
  letter-spacing: 0.08em;
}

.chapter__note {
  font-size: var(--text-xs);
  color: var(--color-ink-muted);
  margin-left: auto;
  font-family: var(--font-mono);
}

/* ============ buttons ============ */
.btn {
  display: inline-flex;
  align-items: center;
  gap: var(--space-1);
  padding: 10px 18px;
  border: var(--border-thin) solid var(--color-line);
  border-radius: var(--radius-md);
  background-color: var(--color-surface);
  color: var(--color-ink);
  font-size: var(--text-sm);
  letter-spacing: 0.05em;
  line-height: 1.2;
  transition: background-color 0.16s ease, color 0.16s ease, border-color 0.16s ease;
}

.btn:hover {
  border-color: var(--color-primary);
  color: var(--color-primary);
}

.btn--primary {
  background-color: var(--color-primary);
  border-color: var(--color-primary);
  color: var(--color-primary-ink);
}

.btn--primary:hover {
  background-color: color-mix(in srgb, var(--color-primary) 84%, #000000);
  border-color: color-mix(in srgb, var(--color-primary) 84%, #000000);
  color: var(--color-primary-ink);
}

.btn--accent {
  background-color: var(--color-accent);
  border-color: var(--color-accent);
  color: var(--color-accent-ink);
}

.btn--accent:hover {
  background-color: color-mix(in srgb, var(--color-accent) 84%, #000000);
  border-color: color-mix(in srgb, var(--color-accent) 84%, #000000);
  color: var(--color-accent-ink);
}

.btn--ghost {
  background-color: transparent;
  border-color: var(--color-line);
  color: var(--color-ink-soft);
}

.btn--ghost:hover {
  border-color: var(--color-primary);
  color: var(--color-primary);
  background-color: var(--color-primary-soft);
}

.btn--block {
  width: 100%;
  justify-content: center;
}

/* ============ tags ============ */
.tag {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  padding: 2px 8px;
  border: var(--border-thin) solid var(--color-line);
  border-radius: var(--radius-sm);
  background-color: var(--color-surface-deep);
  color: var(--color-ink-soft);
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  line-height: 1.6;
  white-space: nowrap;
}

.tag--primary {
  background-color: var(--color-primary-soft);
  border-color: transparent;
  color: var(--color-primary);
}

.tag--accent {
  background-color: var(--color-accent);
  border-color: transparent;
  color: var(--color-accent-ink);
}

.tag--highlight {
  background-color: var(--color-highlight);
  border-color: transparent;
  color: var(--color-accent);
}

.tag--plain {
  background-color: transparent;
  border-color: var(--color-line);
  color: var(--color-ink-muted);
}

/* ============ panels ============ */
.panel {
  background-color: var(--color-surface);
  border: var(--border-thin) solid var(--color-line);
  border-radius: var(--radius-lg);
  overflow: hidden;
}

.panel--muted {
  background-color: var(--color-surface-alt);
}

.panel__head {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  padding: 10px var(--space-2);
  background-color: var(--color-surface-deep);
  border-bottom: var(--border-thin) solid var(--color-line);
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  letter-spacing: 0.08em;
  color: var(--color-ink-soft);
}

.panel__body {
  padding: var(--space-3);
  font-size: var(--text-sm);
}

.panel__foot {
  padding: 10px var(--space-2);
  border-top: var(--border-thin) dashed var(--color-line);
  font-size: var(--text-xs);
  color: var(--color-ink-muted);
}

/* ============ metrics ============ */
.metrics {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
  border-top: var(--border-thin) solid var(--color-line);
  border-left: var(--border-thin) solid var(--color-line);
}

.metric {
  padding: var(--space-3) var(--space-2);
  border-right: var(--border-thin) solid var(--color-line);
  border-bottom: var(--border-thin) solid var(--color-line);
  background-color: var(--color-surface);
}

.metric__label {
  display: block;
  font-size: var(--text-xs);
  color: var(--color-ink-muted);
  letter-spacing: 0.06em;
  margin-bottom: 6px;
}

.metric__value {
  display: block;
  font-family: var(--font-mono);
  font-size: var(--text-2xl);
  line-height: 1.05;
  color: var(--color-primary);
  font-variant-numeric: tabular-nums;
}

.metric--hero .metric__value {
  font-size: var(--text-figure);
  line-height: 1;
}

.metric__unit {
  font-size: var(--text-xs);
  color: var(--color-ink-muted);
  margin-left: 4px;
}

.band-deep .metrics,
.band-deep .metric {
  border-color: var(--color-contrast-line);
}

.band-deep .metric {
  background-color: transparent;
}

.band-deep .metric__label {
  color: var(--color-ink-muted);
}

.band-deep .metric__value {
  color: var(--color-contrast-ink);
}

.band-deep .metric--accent .metric__value {
  color: var(--color-contrast-accent);
}

/* ============ tables ============ */
.table {
  font-size: var(--text-sm);
  border-top: var(--border-thin) solid var(--color-line);
}

.table caption {
  text-align: left;
  padding: var(--space-2) 0;
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  color: var(--color-ink-muted);
  letter-spacing: 0.06em;
}

.table th,
.table td {
  padding: 10px var(--space-2);
  text-align: left;
  vertical-align: top;
  border-bottom: var(--border-thin) solid var(--color-line);
}

.table thead th {
  background-color: var(--color-surface-deep);
  font-family: var(--font-mono);
  font-weight: var(--weight-body);
  font-size: var(--text-xs);
  letter-spacing: 0.08em;
  color: var(--color-ink-soft);
  white-space: nowrap;
}

.table tbody tr:hover {
  background-color: var(--color-primary-soft);
}

.table td.is-num {
  font-family: var(--font-mono);
  font-variant-numeric: tabular-nums;
  white-space: nowrap;
}

.table .is-change {
  color: var(--color-accent);
}

/* ============ row list ============ */
.row-list {
  border-top: var(--border-thin) solid var(--color-line);
}

.row-item {
  position: relative;
  display: flex;
  flex-wrap: wrap;
  align-items: baseline;
  gap: var(--space-1) var(--space-2);
  padding: var(--space-2) var(--space-2) var(--space-2) var(--space-3);
  border-bottom: var(--border-thin) solid var(--color-line);
  transition: background-color 0.16s ease;
}

.row-item__bar {
  position: absolute;
  left: 0;
  top: 0;
  bottom: 0;
  width: 0;
  background-color: var(--color-primary);
  transition: width 0.16s ease;
}

.row-item:hover {
  background-color: var(--color-primary-soft);
}

.row-item:hover .row-item__bar {
  width: 3px;
}

.row-item__no {
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  color: var(--color-ink-muted);
  min-width: 62px;
}

.row-item__name {
  font-size: var(--text-base);
  color: var(--color-ink);
}

.row-item__meta {
  margin-left: auto;
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  color: var(--color-ink-muted);
  font-variant-numeric: tabular-nums;
}

/* ============ breadcrumb ============ */
.breadcrumb {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: var(--space-1);
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  color: var(--color-ink-muted);
  padding-block: var(--space-2);
}

.breadcrumb__sep {
  color: var(--color-line);
}

.breadcrumb a:hover {
  color: var(--color-primary);
}

/* ============ figures ============ */
.figure {
  margin: 0;
  background-color: var(--color-surface-alt);
  border: var(--border-thin) solid var(--color-line);
  border-radius: var(--radius-lg);
  overflow: hidden;
}

.figure__frame {
  position: relative;
  display: block;
  width: 100%;
  aspect-ratio: 16 / 9;
  background-color: var(--color-surface-deep);
}

.figure__img {
  width: 100%;
  height: 100%;
  object-fit: cover;
}

.figure--wide .figure__frame { aspect-ratio: 21 / 9; }
.figure--tall .figure__frame { aspect-ratio: 3 / 4; }
.figure--square .figure__frame { aspect-ratio: 1 / 1; }
.figure--strip .figure__frame { aspect-ratio: 5 / 2; }

.figure__caption {
  padding: 10px var(--space-2);
  border-top: var(--border-thin) dashed var(--color-line);
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  color: var(--color-ink-muted);
  line-height: 1.6;
}

.figure--overlay .figure__scrim {
  position: absolute;
  inset: 0;
  background-image: var(--overlay-scrim);
  z-index: 1;
}

.figure--overlay .figure__overlay {
  position: absolute;
  inset: auto 0 0 0;
  z-index: 2;
  padding: var(--space-3);
  color: var(--color-contrast-ink);
}

.figure--overlay .figure__overlay h3,
.figure--overlay .figure__overlay p {
  color: var(--color-contrast-ink);
}

.figure--deep {
  background-color: var(--color-contrast-surface);
  border-color: var(--color-contrast-line);
}

.figure__grid {
  position: absolute;
  inset: 0;
  z-index: 1;
  pointer-events: none;
  background-image:
    repeating-linear-gradient(to right, rgba(227, 229, 231, 0.07) 0 1px, transparent 1px 48px),
    repeating-linear-gradient(to bottom, rgba(227, 229, 231, 0.07) 0 1px, transparent 1px 48px);
}

/* ============ site header ============ */
.site-header {
  position: sticky;
  top: 0;
  z-index: 120;
  background-color: var(--color-surface);
  border-top: 3px solid var(--color-primary);
  border-bottom: var(--border-thin) solid var(--color-line);
}

.header-rail {
  background-color: var(--color-surface-deep);
  border-bottom: var(--border-thin) dashed var(--color-line);
}

.header-rail__inner {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  min-height: 34px;
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  letter-spacing: 0.04em;
}

.header-rail__mark {
  display: inline-flex;
  align-items: center;
  padding: 2px 7px;
  border-radius: var(--radius-sm);
  background-color: var(--color-primary);
  color: var(--color-primary-ink);
  letter-spacing: 0.1em;
}

.header-rail__meta {
  color: var(--color-ink-muted);
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
}

.header-rail__utility {
  margin-left: auto;
  color: var(--color-primary);
  border-bottom: var(--border-thin) dashed var(--color-primary);
  padding-bottom: 1px;
  white-space: nowrap;
  transition: color 0.16s ease, border-color 0.16s ease;
}

.header-rail__utility:hover {
  color: var(--color-accent);
  border-bottom-color: var(--color-accent);
}

.header-main {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  padding-block: 12px;
}

.header-brand {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  flex: 0 0 auto;
}

.header-brand__mark {
  display: inline-flex;
  align-items: center;
  padding: 6px 8px;
  border-radius: var(--radius-sm);
  background-color: var(--color-primary);
  color: var(--color-primary-ink);
  font-family: var(--font-mono);
  font-size: var(--text-sm);
  letter-spacing: 0.1em;
  line-height: 1;
}

.header-brand__text {
  display: block;
}

.header-brand__name {
  display: block;
  font-family: var(--font-heading);
  font-size: var(--text-lg);
  font-weight: var(--weight-heading);
  letter-spacing: 0.08em;
  line-height: 1.2;
  color: var(--color-ink);
}

.header-brand__sub {
  display: block;
  font-size: var(--text-xs);
  color: var(--color-ink-muted);
  letter-spacing: 0.04em;
  line-height: 1.5;
}

.site-nav {
  margin-left: auto;
  padding-left: var(--space-2);
  border-left: var(--border-thin) dashed var(--color-line);
}

.site-nav__list {
  display: flex;
  align-items: stretch;
  flex-wrap: wrap;
}

.site-nav__link {
  display: flex;
  align-items: center;
  gap: 6px;
  padding: 9px 13px;
  border-left: var(--border-thin) solid var(--color-line);
  font-size: var(--text-sm);
  color: var(--color-ink-soft);
  letter-spacing: 0.05em;
  line-height: 1.2;
  transition: background-color 0.16s ease, color 0.16s ease;
}

.site-nav__item:first-child .site-nav__link {
  border-left: 0;
}

.site-nav__idx {
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  color: var(--color-ink-muted);
  letter-spacing: 0;
}

.site-nav__link:hover {
  background-color: var(--color-surface-alt);
  color: var(--color-ink);
}

.site-nav__link[aria-current="page"] {
  background-color: var(--color-primary);
  color: var(--color-primary-ink);
}

.site-nav__link[aria-current="page"] .site-nav__idx {
  color: var(--color-primary-ink);
  opacity: 0.7;
}

.nav-toggle {
  display: none;
  align-items: center;
  gap: var(--space-1);
  margin-left: auto;
  padding: 9px 14px;
  border: var(--border-thin) solid var(--color-line);
  border-radius: var(--radius-md);
  background-color: var(--color-surface);
  font-size: var(--text-sm);
  letter-spacing: 0.08em;
  color: var(--color-ink-soft);
}

.nav-toggle:hover {
  border-color: var(--color-primary);
  color: var(--color-primary);
}

.nav-toggle__bars {
  position: relative;
  display: block;
  width: 16px;
  height: 11px;
}

.nav-toggle__bars::before,
.nav-toggle__bars::after {
  content: "";
  position: absolute;
  left: 0;
  width: 100%;
  height: 1.5px;
  background-color: currentColor;
  transition: transform 0.16s ease;
}

.nav-toggle__bars::before { top: 0; }
.nav-toggle__bars::after { bottom: 0; }

.nav-toggle[aria-expanded="true"] .nav-toggle__bars::before {
  transform: translateY(4.75px) rotate(45deg);
}

.nav-toggle[aria-expanded="true"] .nav-toggle__bars::after {
  transform: translateY(-4.75px) rotate(-45deg);
}

.header-progress {
  height: 2px;
  background-color: var(--color-line);
}

.header-progress__bar {
  display: block;
  width: 0;
  height: 100%;
  background-color: var(--color-contrast-accent);
}

/* ============ site footer ============ */
.site-footer {
  background-color: var(--color-contrast-surface);
  color: var(--color-contrast-ink);
  border-top: 3px solid var(--color-primary);
}

.footer-top {
  display: grid;
  grid-template-columns: minmax(0, 1fr) minmax(0, 1.8fr);
  gap: var(--space-6);
  padding-block: var(--space-8) var(--space-4);
  border-bottom: var(--border-thin) dashed var(--color-contrast-line);
}

.footer-brand__mark {
  display: inline-flex;
  align-items: center;
  padding: 5px 8px;
  border: var(--border-thin) solid var(--color-contrast-line);
  border-radius: var(--radius-sm);
  font-family: var(--font-mono);
  font-size: var(--text-sm);
  letter-spacing: 0.12em;
  color: var(--color-contrast-accent);
  line-height: 1;
}

.footer-brand__name {
  margin-top: var(--space-2);
  font-family: var(--font-heading);
  font-size: var(--text-xl);
  font-weight: var(--weight-heading);
  letter-spacing: 0.1em;
  color: var(--color-contrast-ink);
}

.footer-brand__sub {
  margin-top: var(--space-1);
  max-width: 34ch;
  font-size: var(--text-xs);
  color: var(--color-ink-muted);
  line-height: 1.8;
}

.footer-cols {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: var(--space-3);
}

.footer-col__title {
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  font-weight: var(--weight-body);
  letter-spacing: 0.14em;
  color: var(--color-contrast-accent);
  padding-bottom: var(--space-1);
  border-bottom: var(--border-thin) dashed var(--color-contrast-line);
  margin-bottom: var(--space-2);
}

.footer-col__list {
  display: grid;
  gap: 10px;
}

.footer-col__link {
  font-size: var(--text-sm);
  color: var(--color-contrast-ink);
  border-bottom: var(--border-thin) solid transparent;
  padding-bottom: 1px;
  transition: color 0.16s ease, border-color 0.16s ease;
}

.footer-col__link:hover {
  color: var(--color-contrast-accent);
  border-bottom-color: var(--color-contrast-accent);
}

.footer-meta {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: var(--space-3);
  padding-block: var(--space-4);
  border-bottom: var(--border-thin) dashed var(--color-contrast-line);
}

.footer-meta__k {
  display: block;
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  letter-spacing: 0.1em;
  color: var(--color-ink-muted);
  margin-bottom: 6px;
}

.footer-meta__v {
  display: block;
  font-size: var(--text-sm);
  color: var(--color-contrast-ink);
  word-break: break-word;
}

.footer-legal {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: space-between;
  gap: var(--space-2);
  padding-block: var(--space-3) var(--space-6);
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  color: var(--color-ink-muted);
}

.to-top {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  padding: 6px 12px;
  border: var(--border-thin) solid var(--color-contrast-line);
  border-radius: var(--radius-md);
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  letter-spacing: 0.08em;
  color: var(--color-contrast-ink);
  transition: color 0.16s ease, border-color 0.16s ease;
}

.to-top:hover {
  color: var(--color-contrast-accent);
  border-color: var(--color-contrast-accent);
}

/* ============ chapter outline ============ */
.outline {
  position: sticky;
  top: 132px;
  display: grid;
  gap: 2px;
}

.outline__link {
  display: flex;
  align-items: baseline;
  gap: var(--space-1);
  padding: 7px 10px;
  border-left: 2px solid var(--color-line);
  font-size: var(--text-sm);
  color: var(--color-ink-muted);
  transition: color 0.16s ease, border-color 0.16s ease, background-color 0.16s ease;
}

.outline__link:hover {
  color: var(--color-ink);
  border-left-color: var(--color-primary);
  background-color: var(--color-surface-alt);
}

.outline__link.is-active {
  color: var(--color-primary);
  border-left-color: var(--color-primary);
  background-color: var(--color-primary-soft);
}

.outline__num {
  font-family: var(--font-mono);
  font-size: var(--text-xs);
}

/* ============ responsive ============ */
@media (max-width: 1180px) {
  .site-nav__link {
    padding: 9px 10px;
    font-size: var(--text-xs);
  }
  .header-brand__sub { display: none; }
}

@media (max-width: 1023px) {
  .grid { gap: var(--space-2); }
  .col-3, .col-4, .col-5, .col-7, .col-9 { grid-column: span 6; }
  .footer-top {
    grid-template-columns: minmax(0, 1fr);
    gap: var(--space-4);
    padding-block: var(--space-6) var(--space-4);
  }
  .footer-meta { grid-template-columns: minmax(0, 1fr); gap: var(--space-2); }
  .outline { position: static; }
}

@media (max-width: 899px) {
  .nav-toggle { display: inline-flex; }

  .site-nav {
    position: fixed;
    top: 0;
    right: 0;
    bottom: 0;
    width: min(320px, 84vw);
    margin-left: 0;
    padding: 92px var(--space-3) var(--space-4);
    background-color: var(--color-contrast-surface);
    border-left: var(--border-thin) solid var(--color-contrast-line);
    border-top: 3px solid var(--color-primary);
    transform: translateX(102%);
    transition: transform 0.2s ease;
    overflow-y: auto;
    z-index: 110;
  }

  .site-nav[data-open] {
    transform: translateX(0);
  }

  .site-nav__list {
    flex-direction: column;
    flex-wrap: nowrap;
  }

  .site-nav__item { width: 100%; }

  .site-nav__link {
    display: flex;
    width: 100%;
    padding: 14px 10px;
    border-left: 0;
    border-bottom: var(--border-thin) dashed var(--color-contrast-line);
    border-radius: 0;
    font-size: var(--text-base);
    color: var(--color-contrast-ink);
  }

  .site-nav__idx { color: var(--color-contrast-accent); }

  .site-nav__link:hover {
    background-color: rgba(255, 255, 255, 0.05);
    color: var(--color-contrast-ink);
  }

  .site-nav__link[aria-current="page"] {
    background-color: var(--color-primary);
    color: var(--color-primary-ink);
  }

  .site-nav__link[aria-current="page"] .site-nav__idx {
    color: var(--color-primary-ink);
    opacity: 0.75;
  }
}

@media (max-width: 767px) {
  .grid { grid-template-columns: minmax(0, 1fr); }
  .col-3, .col-4, .col-5, .col-6, .col-7, .col-8, .col-9, .col-12 { grid-column: 1 / -1; }

  .section { padding-block: var(--space-6); }
  .section--lg { padding-block: var(--space-8); }

  .header-rail__meta { display: none; }
  .header-rail__inner { justify-content: space-between; }

  .chapter__note { margin-left: 0; width: 100%; }

  .footer-cols { grid-template-columns: minmax(0, 1fr); gap: var(--space-3); }
  .footer-legal { flex-direction: column; align-items: flex-start; }

  .metrics { grid-template-columns: repeat(auto-fit, minmax(120px, 1fr)); }
  .figure--wide .figure__frame { aspect-ratio: 16 / 9; }
  .figure--strip .figure__frame { aspect-ratio: 4 / 3; }
}

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

  *,
  *::before,
  *::after {
    animation-duration: 0.001ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.001ms !important;
    scroll-behavior: auto !important;
  }
}