:root {
  --color-surface: #f6f6f6;
  --color-surface-alt: #ededee;
  --color-surface-deep: #e0e1e2;
  --color-ink: #41474e;
  --color-ink-soft: #656e76;
  --color-ink-muted: #777f88;
  --color-line: #d5d6d8;
  --color-primary: #3977b7;
  --color-primary-ink: #ffffff;
  --color-primary-soft: #dce4ed;
  --color-accent: #d92636;
  --color-accent-ink: #ffffff;
  --color-highlight: #f1d1d3;
  --color-focus: #d81829;
  --color-contrast-surface: #161d23;
  --color-contrast-ink: #e4e5e7;
  --color-contrast-line: #43494e;
  --color-contrast-accent: #e15663;
  --overlay-scrim: linear-gradient(180deg, rgba(22, 29, 35, 0.35) 0%, rgba(22, 29, 35, 0.72) 55%, rgba(22, 29, 35, 0.92) 100%);
  --font-heading: "Source Han Serif SC", "Songti SC", Georgia, serif;
  --font-body: "PingFang SC", "Hiragino Sans GB", "Microsoft YaHei", sans-serif;
  --font-mono: ui-monospace, Menlo, Consolas, monospace;
  --text-xs: 9.7px;
  --text-sm: 13.0px;
  --text-base: 15px;
  --text-lg: 20.0px;
  --text-xl: 26.7px;
  --text-2xl: 35.5px;
  --text-3xl: 47.4px;
  --text-4xl: 63.1px;
  --text-display: clamp(50px, 10.3vw, 132px);
  --text-figure: clamp(42px, 7.4vw, 95px);
  --weight-heading: 726;
  --weight-body: 400;
  --leading-body: 1.78;
  --leading-heading: 1.24;
  --tracking-heading: 0.0em;
  --radius-sm: 2px;
  --radius-md: 3px;
  --radius-lg: 4px;
  --radius-pill: 4px;
  --border-thin: 1px;
  --border-thick: 2px;
  --shadow-1: 0 1px 0 rgba(65, 71, 78, 0.12);
  --shadow-2: 0 2px 0 rgba(65, 71, 78, 0.18);
  --space-1: 8px;
  --space-2: 16px;
  --space-3: 24px;
  --space-4: 32px;
  --space-6: 48px;
  --space-8: 64px;
  --container: 1220px;
  --measure: 70ch;
  --gutter: 24px;
  --section-y: 82px;
  --section-y-lg: 115px;
}

/* ============ reset ============ */
*, *::before, *::after { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; text-size-adjust: 100%; }
body {
  margin: 0;
  background: var(--color-surface);
  color: var(--color-ink);
  font-family: var(--font-body);
  font-size: var(--text-base);
  font-weight: var(--weight-body);
  line-height: var(--leading-body);
}
h1, h2, h3, h4, h5, h6 {
  margin: 0 0 var(--space-2);
  font-family: var(--font-heading);
  font-weight: var(--weight-heading);
  line-height: var(--leading-heading);
  letter-spacing: var(--tracking-heading);
  color: var(--color-ink);
}
p { margin: 0 0 var(--space-2); }
ul, ol { margin: 0 0 var(--space-2); padding-left: 1.3em; }
li { margin-bottom: 4px; }
a { color: var(--color-primary); text-decoration-thickness: 1px; text-underline-offset: 3px; }
a:hover { color: var(--color-accent); }
img, svg { max-width: 100%; display: block; }
button { font: inherit; }
strong { font-weight: 600; }
:focus-visible { outline: 2px solid var(--color-focus); outline-offset: 2px; }
[hidden] { display: none !important; }

/* ============ helpers ============ */
.mono { font-family: var(--font-mono); letter-spacing: 0.06em; }

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

.grid {
  display: grid;
  grid-template-columns: repeat(12, minmax(0, 1fr));
  gap: var(--gutter);
}
.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 { margin-bottom: var(--space-3); }
.lede {
  max-width: var(--measure);
  font-size: var(--text-lg);
  line-height: 1.6;
  color: var(--color-ink-soft);
}

.eyebrow {
  display: inline-block;
  margin-bottom: var(--space-1);
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  letter-spacing: 0.24em;
  text-transform: uppercase;
  color: var(--color-accent);
}

.note {
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  letter-spacing: 0.06em;
  color: var(--color-ink-muted);
}
.note--u {
  border-bottom: var(--border-thin) dashed color-mix(in srgb, var(--color-ink-muted) 65%, transparent);
  padding-bottom: 1px;
}
.note[data-tip] { position: relative; cursor: help; }
.note[data-tip]::after {
  content: attr(data-tip);
  position: absolute;
  left: 0;
  top: calc(100% + 6px);
  z-index: 30;
  min-width: 140px;
  max-width: 260px;
  padding: 6px 8px;
  background: var(--color-contrast-surface);
  color: var(--color-contrast-ink);
  font-size: var(--text-xs);
  letter-spacing: 0.04em;
  line-height: 1.6;
  border-radius: var(--radius-sm);
  opacity: 0;
  visibility: hidden;
  transition: opacity 0.15s ease;
}
.note[data-tip]:hover::after,
.note[data-tip]:focus-visible::after { opacity: 1; visibility: visible; }

.scale-rule {
  height: 12px;
  margin: 0;
  border: 0;
  background-image: linear-gradient(to right, var(--color-line) 0 1px, transparent 1px);
  background-size: 14px 5px;
  background-repeat: repeat-x;
  background-position: bottom left;
}
.scale-rule--contrast {
  background-image: linear-gradient(to right, var(--color-contrast-line) 0 1px, transparent 1px);
}

/* ============ buttons ============ */
.btn {
  display: inline-flex;
  align-items: center;
  gap: var(--space-1);
  padding: 10px 18px;
  border: var(--border-thin) solid transparent;
  border-radius: var(--radius-sm);
  background: transparent;
  font-family: var(--font-body);
  font-size: var(--text-sm);
  font-weight: 600;
  letter-spacing: 0.1em;
  line-height: 1.2;
  text-decoration: none;
  cursor: pointer;
  transition: background-color 0.15s ease, color 0.15s ease, transform 0.15s ease, box-shadow 0.15s ease;
}
.btn-primary {
  background: var(--color-accent);
  color: var(--color-accent-ink);
  border-color: var(--color-accent);
}
.btn-primary:hover {
  background: color-mix(in srgb, var(--color-accent) 84%, #000);
  color: var(--color-accent-ink);
  transform: translateY(-1px);
  box-shadow: var(--shadow-2);
}
.btn-outline {
  background: transparent;
  color: var(--color-ink);
  border-color: var(--color-ink);
}
.btn-outline:hover {
  background: var(--color-primary-soft);
  color: var(--color-primary);
  border-color: var(--color-primary);
}
.btn--sm { padding: 7px 12px; font-size: var(--text-xs); }

/* ============ header ============ */
.site-header { position: relative; z-index: 40; }

.skip-link {
  position: absolute;
  left: var(--gutter);
  top: -60px;
  z-index: 90;
  padding: 10px 16px;
  background: var(--color-accent);
  color: var(--color-accent-ink);
  font-size: var(--text-sm);
  text-decoration: none;
  border-radius: var(--radius-sm);
  transition: top 0.15s ease;
}
.skip-link:focus { top: 8px; color: var(--color-accent-ink); }

.header-status {
  background: var(--color-contrast-surface);
  color: var(--color-contrast-ink);
}
.header-status-inner {
  display: flex;
  align-items: center;
  gap: var(--space-6);
  padding-block: 7px;
  overflow-x: auto;
  white-space: nowrap;
  scrollbar-width: none;
}
.header-status-inner::-webkit-scrollbar { display: none; }
.status-item {
  display: inline-flex;
  align-items: baseline;
  gap: 6px;
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  letter-spacing: 0.1em;
}
.status-dot {
  display: inline-block;
  width: 6px;
  height: 6px;
  background: var(--color-contrast-accent);
  align-self: center;
}
.status-key { color: color-mix(in srgb, var(--color-contrast-ink) 62%, transparent); }
.status-value { color: var(--color-contrast-ink); }

.header-bar {
  position: sticky;
  top: 0;
  z-index: 35;
  background: var(--color-surface);
  border-bottom: var(--border-thin) solid var(--color-line);
}
.header-bar-inner {
  display: flex;
  align-items: center;
  flex-wrap: wrap;
  gap: var(--space-2);
  min-height: 74px;
}
.header-progress {
  position: absolute;
  left: 0;
  bottom: -1px;
  width: 100%;
  height: 2px;
  background: var(--color-accent);
  transform: scaleX(0);
  transform-origin: left center;
}

.brand {
  display: inline-flex;
  align-items: center;
  gap: 10px;
  text-decoration: none;
  color: var(--color-ink);
}
.brand:hover { color: var(--color-ink); }
.brand-mark {
  display: grid;
  grid-template-columns: repeat(2, 11px);
  grid-template-rows: repeat(2, 11px);
  gap: 2px;
  flex: none;
}
.brand-mark i { display: block; border-radius: 1px; }
.brand-mark i:nth-child(1) { background: var(--color-primary); }
.brand-mark i:nth-child(2) { background: var(--color-accent); }
.brand-mark i:nth-child(3) { background: var(--color-contrast-surface); }
.brand-mark i:nth-child(4) { background: var(--color-highlight); }
.brand-text { display: flex; flex-direction: column; line-height: 1.1; }
.brand-name {
  font-family: var(--font-heading);
  font-size: var(--text-lg);
  font-weight: var(--weight-heading);
  letter-spacing: 0.08em;
}
.brand-sub {
  font-size: var(--text-xs);
  letter-spacing: 0.22em;
  color: var(--color-ink-muted);
}

.nav-toggle {
  display: none;
  order: 3;
  align-items: center;
  gap: 8px;
  padding: 8px 12px;
  background: var(--color-surface);
  border: var(--border-thin) solid var(--color-line);
  border-radius: var(--radius-sm);
  color: var(--color-ink);
  font-size: var(--text-sm);
  letter-spacing: 0.1em;
  cursor: pointer;
}
.nav-toggle-bars { display: grid; gap: 3px; width: 16px; }
.nav-toggle-bars i { display: block; height: 2px; background: currentColor; }
.nav-toggle[aria-expanded="true"] {
  background: var(--color-primary-soft);
  border-color: var(--color-primary);
  color: var(--color-primary);
}

.nav { margin-left: auto; }
.nav-list {
  display: flex;
  align-items: center;
  gap: 2px;
  margin: 0;
  padding: 0;
  list-style: none;
}
.nav-item { margin: 0; }
.nav-link {
  position: relative;
  display: inline-block;
  padding: 10px 12px;
  font-size: var(--text-sm);
  letter-spacing: 0.04em;
  color: var(--color-ink-soft);
  text-decoration: none;
  border-radius: var(--radius-sm);
  transition: color 0.15s ease, background-color 0.15s ease;
}
.nav-link::before {
  content: "";
  position: absolute;
  left: 4px;
  top: 50%;
  width: 2px;
  height: 0;
  background: var(--color-accent);
  transform: translateY(-50%);
  transition: height 0.18s ease;
}
.nav-link:hover {
  color: var(--color-ink);
  background: var(--color-primary-soft);
}
.nav-link:hover::before { height: 14px; }
.nav-link[aria-current="page"] { color: var(--color-accent); font-weight: 600; }
.nav-link[aria-current="page"]::before { height: 20px; }

.header-cta { order: 2; flex: none; }

/* ============ sections ============ */
.section { padding-block: var(--section-y); }
.section--lg { padding-block: var(--section-y-lg); }
.section--alt { background: var(--color-surface-alt); }
.section--deep { background: var(--color-surface-deep); }
.section--dark {
  background: var(--color-contrast-surface);
  color: var(--color-contrast-ink);
}
.section--dark h1, .section--dark h2, .section--dark h3, .section--dark h4 { color: var(--color-contrast-ink); }
.section--dark a { color: var(--color-contrast-ink); }
.section--dark .section-note { color: color-mix(in srgb, var(--color-contrast-ink) 68%, transparent); }
.section--dark .lede { color: color-mix(in srgb, var(--color-contrast-ink) 80%, transparent); }
.section--dark .tag {
  background: transparent;
  color: var(--color-contrast-ink);
  border-color: var(--color-contrast-line);
}

.section-head {
  display: flex;
  flex-wrap: wrap;
  align-items: baseline;
  gap: var(--space-1) var(--space-3);
  margin-bottom: var(--space-4);
}
.section-index {
  font-family: var(--font-mono);
  font-size: var(--text-sm);
  letter-spacing: 0.18em;
  color: var(--color-accent);
}
.section-title {
  margin: 0;
  font-size: var(--text-2xl);
  letter-spacing: 0.02em;
}
.section-note {
  margin-left: auto;
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  letter-spacing: 0.08em;
  color: var(--color-ink-muted);
}

/* ============ tag ============ */
.tag {
  display: inline-block;
  padding: 3px 8px;
  border: var(--border-thin) solid var(--color-line);
  border-radius: var(--radius-sm);
  background: var(--color-surface);
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--color-ink-soft);
}
.tag--primary { background: var(--color-primary); color: var(--color-primary-ink); border-color: var(--color-primary); }
.tag--accent { background: var(--color-accent); color: var(--color-accent-ink); border-color: var(--color-accent); }
.tag--soft { background: var(--color-primary-soft); color: var(--color-primary); border-color: transparent; }

/* ============ panel ============ */
.panel {
  border: var(--border-thin) solid var(--color-line);
  border-radius: var(--radius-sm);
  background: var(--color-surface);
}
.panel + .panel { margin-top: -1px; }
.panel-head {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: var(--space-2);
  padding: var(--space-2) var(--space-3);
  background: var(--color-primary-soft);
  font-family: var(--font-body);
  font-size: var(--text-sm);
  font-weight: 600;
  letter-spacing: 0.06em;
  color: var(--color-ink);
  cursor: pointer;
  list-style: none;
}
.panel-head::-webkit-details-marker { display: none; }
.panel-head::after {
  content: "+";
  font-family: var(--font-mono);
  font-size: var(--text-sm);
  color: var(--color-primary);
}
.panel[open] > .panel-head { border-bottom: var(--border-thin) solid var(--color-line); }
.panel[open] > .panel-head::after { content: "−"; }
.panel-body {
  padding: var(--space-3);
  background: var(--color-surface);
}
.panel-body > :last-child { margin-bottom: 0; }
.section--dark .panel { background: transparent; border-color: var(--color-contrast-line); }
.section--dark .panel-head { background: transparent; color: var(--color-contrast-ink); }
.section--dark .panel[open] > .panel-head { border-bottom-color: var(--color-contrast-line); }
.section--dark .panel-body { background: transparent; color: var(--color-contrast-ink); }

/* ============ media ============ */
.media {
  position: relative;
  display: block;
  margin: 0;
  overflow: hidden;
  background: var(--color-surface-deep);
  border: var(--border-thin) solid var(--color-line);
  border-radius: var(--radius-sm);
}
.media-img, .media > img {
  display: block;
  width: 100%;
  height: 100%;
  object-fit: cover;
}
.media--wide { aspect-ratio: 16 / 9; }
.media--tall { aspect-ratio: 3 / 4; }
.media--square { aspect-ratio: 1 / 1; }
.media-scrim {
  position: absolute;
  inset: 0;
  z-index: 1;
  background: var(--overlay-scrim);
  pointer-events: none;
}
.media-body {
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  z-index: 2;
  padding: var(--space-3);
  color: var(--color-contrast-ink);
}
.media-body h2, .media-body h3 { color: var(--color-contrast-ink); margin-bottom: var(--space-1); }
.media-body p {
  margin: 0;
  font-size: var(--text-sm);
  color: color-mix(in srgb, var(--color-contrast-ink) 82%, transparent);
}
.media-body .note { color: color-mix(in srgb, var(--color-contrast-ink) 70%, transparent); }

/* ============ breadcrumb ============ */
.breadcrumb { margin-bottom: var(--space-3); }
.breadcrumb-list {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: var(--space-1);
  margin: 0;
  padding: 0;
  list-style: none;
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  letter-spacing: 0.1em;
  color: var(--color-ink-muted);
}
.breadcrumb-item { display: inline-flex; align-items: center; gap: var(--space-1); margin: 0; }
.breadcrumb-item + .breadcrumb-item::before { content: "/"; color: var(--color-line); }
.breadcrumb-item a { color: var(--color-ink-muted); text-decoration: none; }
.breadcrumb-item a:hover { color: var(--color-accent); text-decoration: underline; }
.breadcrumb-item[aria-current="page"] { color: var(--color-ink); }

/* ============ section link (page-level toc) ============ */
[data-section-link] {
  display: block;
  padding: 6px 0 6px 10px;
  border-left: var(--border-thick) solid transparent;
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  letter-spacing: 0.08em;
  color: var(--color-ink-muted);
  text-decoration: none;
  transition: color 0.15s ease, border-color 0.15s ease;
}
[data-section-link]:hover { color: var(--color-ink); border-left-color: var(--color-line); }
[data-section-link][aria-current="true"] { color: var(--color-accent); border-left-color: var(--color-accent); }

/* ============ footer ============ */
.site-footer {
  background: var(--color-contrast-surface);
  color: var(--color-contrast-ink);
}
.footer-inner { padding-block: var(--space-8) var(--space-6); }
.footer-top {
  display: grid;
  grid-template-columns: minmax(0, 0.9fr) minmax(0, 1.1fr);
  gap: var(--space-6);
  align-items: start;
}
.footer-quad {
  display: grid;
  grid-template-columns: repeat(4, 20px);
  gap: 3px;
  margin-bottom: var(--space-2);
}
.footer-quad i { display: block; height: 8px; }
.footer-quad i:nth-child(1) { background: var(--color-primary); }
.footer-quad i:nth-child(2) { background: var(--color-accent); }
.footer-quad i:nth-child(3) { background: var(--color-highlight); }
.footer-quad i:nth-child(4) { background: var(--color-contrast-accent); }
.footer-title {
  margin: 0 0 var(--space-1);
  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-line {
  margin: 0;
  font-size: var(--text-xs);
  letter-spacing: 0.08em;
  color: color-mix(in srgb, var(--color-contrast-ink) 66%, transparent);
}
.footer-nav { justify-self: end; }
.footer-scale {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-2) var(--space-6);
  margin: 0;
  padding: 0;
  list-style: none;
}
.footer-scale-item {
  display: flex;
  align-items: baseline;
  gap: 8px;
  margin: 0;
}
.footer-scale-num {
  font-size: var(--text-xs);
  letter-spacing: 0.14em;
  color: var(--color-contrast-accent);
}
.footer-nav-link {
  padding-bottom: 2px;
  border-bottom: var(--border-thin) dashed transparent;
  color: var(--color-contrast-ink);
  font-size: var(--text-sm);
  letter-spacing: 0.06em;
  text-decoration: none;
  transition: border-color 0.15s ease, color 0.15s ease;
}
.footer-nav-link:hover {
  color: var(--color-primary-ink);
  border-bottom-color: var(--color-contrast-accent);
}
.footer-nav-link[aria-current="page"] { color: var(--color-contrast-accent); }
.footer-rule {
  height: 12px;
  margin-block: var(--space-6) var(--space-3);
  background-image: linear-gradient(to right, var(--color-contrast-line) 0 1px, transparent 1px);
  background-size: 18px 8px;
  background-repeat: repeat-x;
  background-position: top left;
}
.footer-meta {
  display: flex;
  flex-wrap: wrap;
  align-items: baseline;
  justify-content: space-between;
  gap: var(--space-1) var(--space-6);
}
.footer-meta p {
  margin: 0;
  font-size: var(--text-xs);
  letter-spacing: 0.06em;
  color: color-mix(in srgb, var(--color-contrast-ink) 72%, transparent);
}
.footer-meta .footer-contact { color: var(--color-contrast-ink); }

/* ============ responsive ============ */
@media (max-width: 1080px) {
  .nav-link { padding: 10px 8px; letter-spacing: 0.02em; }
}

@media (max-width: 900px) {
  .header-bar-inner { min-height: 66px; }
  .nav-toggle { display: inline-flex; order: 3; }
  .brand { order: 1; }
  .header-cta { order: 2; margin-left: auto; }
  .nav {
    order: 4;
    flex-basis: 100%;
    margin-left: 0;
    display: none;
    border-top: var(--border-thin) solid var(--color-line);
  }
  .nav[data-open="true"] { display: block; }
  .nav-list {
    flex-wrap: nowrap;
    gap: 0;
    padding-block: var(--space-1);
    overflow-x: auto;
    scrollbar-width: none;
  }
  .nav-list::-webkit-scrollbar { display: none; }
  .nav-link { white-space: nowrap; padding: 10px 14px; }
  .nav-link::before { left: 2px; }

  .grid { grid-template-columns: repeat(6, minmax(0, 1fr)); }
  .col-3, .col-4, .col-5, .col-6, .col-7, .col-8, .col-9, .col-12 { grid-column: span 6; }

  .section-note { margin-left: 0; flex-basis: 100%; }
  .footer-top { grid-template-columns: 1fr; gap: var(--space-4); }
  .footer-nav { justify-self: start; }
}

@media (max-width: 640px) {
  .container { padding-inline: var(--space-2); }
  .header-status-inner { gap: var(--space-3); }
  .brand-sub { display: none; }
  .header-cta { padding: 8px 12px; font-size: var(--text-xs); letter-spacing: 0.08em; }
  .section { padding-block: var(--space-6); }
  .section--lg { padding-block: var(--space-8); }
  .section-title { font-size: var(--text-xl); }
  .grid { grid-template-columns: 1fr; }
  .col-3, .col-4, .col-5, .col-6, .col-7, .col-8, .col-9, .col-12 { grid-column: span 1; }
  .footer-meta { flex-direction: column; gap: var(--space-1); }
  .footer-scale { gap: var(--space-2) var(--space-4); }
}

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0.001ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.001ms !important;
    scroll-behavior: auto !important;
  }
  .btn-primary:hover { transform: none; }
  .header-progress { transition: none; }
}