/* =========================================================================
   Layout — containers, section surfaces, grids, header, footer.
   ========================================================================= */

.container {
  width: 100%;
  max-width: calc(var(--container) + 2 * var(--gutter));
  margin-inline: auto;
  padding-inline: var(--gutter);
}
.container--narrow { max-width: calc(var(--container-narrow) + 2 * var(--gutter)); }

/* ---- Sections and their surfaces --------------------------------------- */
/* Every section carries its own full padding. Surfaces alternate page by
   page, so two white sections in a row are told apart by a hairline
   (.section-line) rather than by collapsing their padding. */
.section { padding-block: var(--section-pad); }
.section--tight { padding-block: calc(var(--section-pad) * 0.55); }
.section-line { border-top: 1px solid var(--line); }

.section--dawn { background: var(--dawn); }
.section--mist { background: var(--mist); }
.section--sun  { background: var(--sun); }
.section--sea  { background: var(--sea); }

/* A dark surface flips the tokens its children read, so every component
   (type, rules, buttons, lists) follows without a second set of rules. */
.on-dark {
  --ink: #ffffff;
  --ink-soft: var(--on-dark-soft);
  --ink-faint: var(--on-dark-faint);
  --line: var(--line-on-dark);
  --line-strong: rgba(255, 255, 255, 0.5);
  --line-faint: rgba(255, 255, 255, 0.12);
  --btn-primary-bg: var(--sun);
  --btn-primary-bg-hover: #ffd75e;
  --btn-primary-ink: #141414;
  color: var(--ink);
}
.on-dark :focus-visible { outline-color: var(--sun); }

/* On the sun surface a line of sun would vanish: links are ruled in ink. */
.section--sun,
.band--sun,
.page-hero--sun {
  --link-line: var(--ink);
  --link-hover-ink: var(--sun);
  --line: rgba(20, 20, 20, 0.22);
}

/* ---- Section heading --------------------------------------------------- */
.section-head { margin-bottom: var(--space-6); }
.section-head__title { font-size: var(--step-3); max-width: 26ch; }
.section-head__index {
  display: block;
  margin-bottom: var(--space-3);
  font-family: var(--font-ui);
  font-size: var(--step--1);
  font-weight: 600;
  color: var(--ink-soft);
}

/* ---- Chapter: heading in the margin, prose beside it -------------------- */
.chapter { display: grid; gap: var(--space-5) var(--space-8); }
.chapter__head > * + * { margin-top: var(--space-4); }
@media (min-width: 900px) {
  .chapter { grid-template-columns: minmax(0, 5fr) minmax(0, 7fr); align-items: start; }
  .chapter__head { position: sticky; top: calc(var(--space-6)); }
}

/* ---- Split: content and a narrower aside -------------------------------- */
.split { display: grid; grid-template-columns: minmax(0, 1fr); gap: clamp(var(--space-6), 5vw, var(--space-9)); }
@media (min-width: 940px) {
  .split { grid-template-columns: minmax(0, 1fr) minmax(300px, 380px); align-items: start; }
  .split--aside-left { grid-template-columns: minmax(300px, 380px) minmax(0, 1fr); }
  .split--even { grid-template-columns: minmax(0, 1fr) minmax(0, 1fr); }
  .split__aside { position: sticky; top: var(--space-6); }
}

/* ---- Helpers ------------------------------------------------------------- */
.stack > * + * { margin-top: var(--space-4); }
.stack-lg > * + * { margin-top: var(--space-6); }
.grid { display: grid; gap: var(--space-6); }
.grid-2 { grid-template-columns: repeat(2, minmax(0, 1fr)); }
.grid-3 { grid-template-columns: repeat(3, minmax(0, 1fr)); }
.grid-auto { grid-template-columns: repeat(auto-fill, minmax(280px, 1fr)); }
.cluster { display: flex; flex-wrap: wrap; gap: var(--space-3); align-items: center; }
@media (max-width: 760px) { .grid-2, .grid-3 { grid-template-columns: minmax(0, 1fr); } }

/* ---- Header ---------------------------------------------------------------- */
.site-header {
  position: sticky;
  top: 0;
  z-index: var(--z-header);
  background: rgba(255, 255, 255, 0.92);
  -webkit-backdrop-filter: saturate(140%) blur(14px);
  backdrop-filter: saturate(140%) blur(14px);
  border-bottom: 1px solid transparent;
  font-family: var(--font-ui);
  transition: transform 320ms var(--ease), background var(--dur) var(--ease),
              color var(--dur) var(--ease), border-color var(--dur) var(--ease);
}
.site-header.is-stuck { border-bottom-color: var(--line); }
.site-header.is-hidden { transform: translateY(-101%); }
.site-header__inner {
  display: flex;
  align-items: center;
  gap: var(--space-5);
  min-height: var(--header-h);
}

/* Over a dark hero the header floats clear, in white, until the page moves.
   Needs the script (it removes the state); without it the header stays a
   plain white bar, which is always legible. */
.js .hero-dark .site-header {
  position: fixed;
  inset: 0 0 auto 0;
}
.js .hero-dark .site-header:not(.is-stuck) {
  background: transparent;
  -webkit-backdrop-filter: none;
  backdrop-filter: none;
  color: var(--on-dark);
  --ink: #ffffff;
  --ink-soft: rgba(255, 255, 255, 0.86);
  --btn-primary-bg: var(--sun);
  --btn-primary-bg-hover: #ffd75e;
  --btn-primary-ink: #141414;
}

/* A flash message on such a page must clear the floating bar. */
.js .hero-dark main > .container:first-child { padding-top: calc(var(--header-h) + var(--space-4)); }

.brand { display: inline-flex; align-items: baseline; gap: var(--space-2); margin-right: auto; }
.brand__name {
  font-family: var(--font-display);
  font-size: 1.32rem;
  font-weight: 700;
  font-stretch: 88%;
  letter-spacing: -0.02em;
  line-height: 1;
}

.nav { display: flex; align-items: center; gap: var(--space-5); }
.nav__list { display: flex; align-items: center; gap: clamp(var(--space-4), 2.2vw, var(--space-6)); }
.nav__link {
  position: relative;
  padding-block: var(--space-2);
  font-size: 0.98rem;
  font-weight: 500;
  color: var(--ink-soft);
  transition: color var(--dur) var(--ease);
}
.nav__link::after {
  content: "";
  position: absolute;
  left: 0; right: 0; bottom: 0;
  height: 3px;
  border-radius: 3px;
  background: var(--sun);
  transform: scaleX(0);
  transform-origin: left;
  transition: transform var(--dur) var(--ease);
}
.nav__link:hover,
.nav__link[aria-current="page"] { color: var(--ink); }
.nav__link:hover::after,
.nav__link[aria-current="page"]::after { transform: scaleX(1); }

.nav__toggle {
  display: none;
  width: 46px; height: 46px;
  align-items: center;
  justify-content: center;
  border: 0;
  border-radius: 50%;
  background: transparent;
  cursor: pointer;
}
.nav__toggle span,
.nav__toggle span::before,
.nav__toggle span::after {
  content: "";
  display: block;
  width: 22px; height: 2px;
  border-radius: 2px;
  background: currentColor;
  transition: transform var(--dur) var(--ease), background var(--dur) var(--ease);
}
.nav__toggle span { position: relative; background: transparent; }
.nav__toggle span::before { position: absolute; top: -4px; }
.nav__toggle span::after  { position: absolute; top: 4px; }
.nav__toggle[aria-expanded="true"] span::before { transform: translateY(4px) rotate(45deg); }
.nav__toggle[aria-expanded="true"] span::after  { transform: translateY(-4px) rotate(-45deg); }

@media (max-width: 900px) {
  .nav__toggle { display: inline-flex; position: relative; z-index: 2; }
  .brand { position: relative; z-index: 2; }
  .nav {
    position: fixed;
    inset: 0;
    z-index: 1;
    flex-direction: column;
    align-items: stretch;
    justify-content: space-between;
    gap: var(--space-6);
    padding: calc(var(--header-h) + var(--space-6)) var(--gutter) var(--space-7);
    height: 100vh;
    height: 100dvh;
    overflow-y: auto;
    background: var(--paper);
    color: #141414;
    visibility: hidden;
    opacity: 0;
    transform: translateY(-2%);
    transition: opacity var(--dur) var(--ease), transform var(--dur) var(--ease), visibility 0s linear var(--dur);
  }
  .nav.is-open {
    visibility: visible;
    opacity: 1;
    transform: none;
    transition: opacity var(--dur) var(--ease), transform var(--dur) var(--ease), visibility 0s;
  }
  .nav__list { flex-direction: column; align-items: stretch; gap: 0; }
  .nav__link {
    display: block;
    padding-block: var(--space-4);
    border-bottom: 1px solid rgba(20, 20, 20, 0.13);
    font-family: var(--font-display);
    font-size: var(--step-2);
    font-weight: 700;
    font-stretch: 88%;
    letter-spacing: -0.02em;
    color: #141414;
  }
  .nav__link::after { display: none; }
  .nav .btn { align-self: stretch; }
  /* While the menu is open the bar is always a white bar in black type. */
  .nav-open .site-header,
  .js .hero-dark.nav-open .site-header:not(.is-stuck) {
    background: var(--paper);
    color: #141414;
    --ink: #141414;
    --btn-primary-bg: #141414;
    --btn-primary-bg-hover: #000000;
    --btn-primary-ink: #ffffff;
    transform: none;
  }
}

/* ---- Footer ------------------------------------------------------------------ */
.site-footer {
  border-top: 1px solid var(--line);
  padding-block: var(--space-9) var(--space-6);
  font-family: var(--font-ui);
}
.footer-mark {
  font-family: var(--font-display);
  font-size: clamp(2.6rem, 1.2rem + 7vw, 7.5rem);
  font-weight: 800;
  font-stretch: 84%;
  letter-spacing: -0.02em;
  line-height: 0.95;
}
.footer-tag { margin-top: var(--space-4); font-family: var(--font-body); font-size: var(--step-1); color: var(--ink-soft); }
.footer-grid {
  display: grid;
  gap: var(--space-7) var(--space-8);
  grid-template-columns: minmax(0, 7fr) minmax(0, 5fr);
  margin-top: var(--space-8);
  padding-top: var(--space-7);
  border-top: 1px solid var(--line);
}
.footer-col p { font-family: var(--font-body); color: var(--ink-soft); max-width: 46ch; }
.footer-col h4 { font-size: var(--step--1); font-weight: 600; color: var(--ink-faint); letter-spacing: 0; margin-bottom: var(--space-4); }
.footer-col ul { display: grid; gap: var(--space-2); }
.footer-col li a { font-size: var(--step-0); font-weight: 500; }
.footer-col li a:hover { text-decoration: underline; text-decoration-color: var(--sun); text-decoration-thickness: 3px; text-underline-offset: 5px; }
.footer-bottom {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: space-between;
  gap: var(--space-4);
  margin-top: var(--space-8);
  font-size: var(--step--1);
  color: var(--ink-faint);
}
@media (max-width: 760px) { .footer-grid { grid-template-columns: minmax(0, 1fr); } }

/* Language switch: two words, the current one set in a sun pill. */
.lang-switch { display: inline-flex; align-items: center; gap: var(--space-1); margin: 0; padding: 4px; border: 1px solid var(--line); border-radius: var(--radius-pill); }
.lang-switch__option {
  appearance: none;
  border: 0;
  border-radius: var(--radius-pill);
  padding: 0.35em 0.95em;
  background: transparent;
  font-family: var(--font-ui);
  font-size: var(--step--1);
  font-weight: 600;
  color: var(--ink-soft);
  cursor: pointer;
  transition: background var(--dur) var(--ease), color var(--dur) var(--ease);
}
.lang-switch__option:hover { color: var(--ink); background: var(--mist); }
.lang-switch__option.is-current { background: var(--sun); color: #141414; cursor: default; }