/* ============================================================================
   TUTORIAL HUB — REFINEMENT LAYER
   ----------------------------------------------------------------------------
   Loaded last. Everything here raises the finish of the reading experience
   without changing the structure underneath, so a lesson written months ago
   picks up the improvements with no edit.

   What this layer is responsible for:
     1. Vertical rhythm. One spacing scale, applied consistently, so a long
        lesson reads as a document rather than a stack of components.
     2. Signature components. The callout, the ladder, the exercise and the
        quiz are what make this feel like a course rather than a blog, so
        they get the detail.
     3. Restraint. No gradient washes, no shadows doing work a border can do,
        no animation that is not communicating state.
   ========================================================================= */

:root {
  --sp-1: 6px;   --sp-2: 10px;  --sp-3: 16px;
  --sp-4: 24px;  --sp-5: 34px;  --sp-6: 48px;  --sp-7: 68px;
  --measure: 70ch;
  --ease: cubic-bezier(.32, .72, .28, 1);
}

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { animation-duration: .01ms !important; transition-duration: .01ms !important; }
}

/* ============================================================ chrome ==== */

.topbar {
  height: var(--topbar);
  background: color-mix(in srgb, var(--bg) 82%, transparent);
  backdrop-filter: saturate(1.6) blur(14px);
  -webkit-backdrop-filter: saturate(1.6) blur(14px);
  border-bottom: 1px solid var(--border-soft);
}
.brand-mark {
  background: linear-gradient(180deg, var(--accent) 0%, color-mix(in srgb, var(--accent) 72%, #000) 100%);
  box-shadow: 0 1px 0 rgba(255,255,255,.14) inset, var(--e1);
  letter-spacing: -.02em;
}

/* Track switch — a segmented control, sized so the label is the target and
   the count reads as secondary. */
.track-switch { display: flex; gap: 3px; padding: 3px; margin: 0 0 12px;
  background: var(--surface-2); border: 1px solid var(--border); border-radius: 10px; }
.tsw {
  flex: 1 1 0; display: flex; align-items: center; justify-content: center; gap: 7px;
  padding: 7px 8px; border-radius: 7px; font-size: 12.4px; font-weight: 600;
  color: var(--ink-3); text-decoration: none; white-space: nowrap;
  transition: color .16s var(--ease), background-color .16s var(--ease);
}
.tsw:hover { color: var(--ink); background: var(--surface-3); text-decoration: none; }
.tsw.on { color: var(--ink); background: var(--bg-elev); box-shadow: var(--e1); }
.tsw.on .tsw-n { background: var(--accent-soft); color: var(--accent-ink); border-color: var(--accent-line); }
.tsw-n {
  font-family: var(--mono); font-size: 9.8px; font-weight: 600; line-height: 1;
  padding: 3px 5px; border-radius: 4px; border: 1px solid var(--border);
  background: var(--surface); color: var(--ink-3);
}

/* ==================================================== lesson header ===== */

.lh { padding: var(--sp-5) 0 0; }

.lh-eyebrow { font-size: 12px; margin-bottom: var(--sp-3); letter-spacing: .01em; }
.lh-eyebrow .lv {
  display: inline-flex; align-items: center; gap: 7px;
  padding: 3px 9px 3px 7px; border-radius: 999px;
  background: var(--accent-soft); border: 1px solid var(--accent-line);
  font-size: 11.2px; letter-spacing: .01em;
}

.lh h1 {
  font-size: clamp(30px, 3.6vw, 42px);
  line-height: 1.13;
  letter-spacing: -.023em;
  font-weight: 750;
  margin: 0 0 var(--sp-3);
  max-width: 22ch;
}
.lh-lede {
  font-size: 18px; line-height: 1.62; color: var(--ink-2);
  margin: 0 0 var(--sp-4); max-width: 64ch;
}
.lh-lede b, .lh-lede strong { color: var(--ink); font-weight: 640; }

/* Meta strip — chips rather than a run-on line, so each fact is its own
   object and the row survives wrapping on a phone. */
.lh-meta {
  display: flex; flex-wrap: wrap; align-items: center; gap: var(--sp-2);
  padding: 0; border: 0; margin: 0 0 var(--sp-2);
}
.lh-meta .meta-row { display: contents; }
.lh-meta .mi {
  display: inline-flex; align-items: center; gap: 6px;
  padding: 5px 10px; border-radius: 8px;
  background: var(--surface); border: 1px solid var(--border);
  font-size: 12.3px; color: var(--ink-3);
}
.lh-meta .mi b { color: var(--ink-2); font-weight: 640; }
.lh-meta .mi svg { width: 13px; height: 13px; opacity: .8; }
.lh-meta .badge { padding: 5px 11px; border-radius: 8px; font-size: 11.6px; }

/* Module progress — where this lesson sits, as segments rather than a
   percentage, because "4 of 9" is the useful unit. */
.lh-prog { display: flex; align-items: center; gap: 10px; margin: var(--sp-3) 0 0; }
.lh-prog-seg { display: flex; gap: 3px; flex: 1 1 auto; max-width: 320px; }
.lh-prog-seg i {
  flex: 1 1 0; height: 4px; border-radius: 2px; background: var(--border);
  transition: background-color .2s var(--ease);
}
.lh-prog-seg i.done { background: var(--accent-line); }
.lh-prog-seg i.cur  { background: var(--accent); }
.lh-prog-t { font-size: 11.8px; color: var(--ink-3); white-space: nowrap; }
.lh-prog-t b { color: var(--ink-2); font-weight: 640; }

/* Contract cards */
.lh-contract { gap: var(--sp-3); margin: var(--sp-4) 0 var(--sp-2); }
.contract-box {
  padding: 17px 19px 18px; border-radius: 14px;
  background: var(--surface); border: 1px solid var(--border);
}
.contract-box.obj {
  border-left: 0;
  background:
    linear-gradient(var(--accent) 0 0) left / 3px 100% no-repeat,
    var(--surface);
  padding-left: 22px;
}
.contract-h { font-size: 10.4px; letter-spacing: .12em; margin-bottom: 13px; }
.contract-box ol li { font-size: 14.1px; padding-left: 27px; margin-bottom: 9px; }
.contract-box ol li::before { font-size: 10px; }
.contract-box ul li { font-size: 13.6px; }

/* ==================================================== body typography === */

.body { padding-top: var(--sp-2); }

.body > h2 {
  position: relative;
  margin-top: var(--sp-7);
  padding-top: var(--sp-5);
  font-size: clamp(21px, 2.1vw, 25px);
  letter-spacing: -.016em;
  line-height: 1.25;
  display: flex; align-items: baseline; gap: 13px;
  border-top: 1px solid var(--border-soft);
}
.body > h2:first-child { margin-top: var(--sp-5); }
.body > h2 .hn {
  font-family: var(--mono); font-size: 11.5px; font-weight: 700;
  color: var(--accent-ink); background: var(--accent-soft);
  border: 1px solid var(--accent-line); border-radius: 7px;
  padding: 4px 8px; line-height: 1; flex: none; transform: translateY(-2px);
}
.body > h2 .hn:empty { display: none; }
.body > h2 + .h2-sub { font-size: 14.6px; margin: 8px 0 var(--sp-4); color: var(--ink-3); }

.body > h3 {
  margin: var(--sp-5) 0 var(--sp-2);
  font-size: 17.5px; letter-spacing: -.012em; font-weight: 660;
}

/* A real anchor, revealed on hover and always reachable by keyboard. */
.anchor {
  margin-left: 10px; font-family: var(--mono); font-weight: 500;
  color: var(--ink-3); opacity: 0; text-decoration: none;
  transition: opacity .15s var(--ease), color .15s var(--ease);
}
h2:hover .anchor, h3:hover .anchor, .anchor:focus-visible { opacity: 1; }
.anchor:hover { color: var(--accent-ink); text-decoration: none; }
:target { scroll-margin-top: calc(var(--topbar) + 22px); }

.body > p { font-size: 16.2px; line-height: 1.68; margin: 0 0 1.05em; }
.body > p, .body > ul, .body > ol { max-width: var(--measure); }
.body li { line-height: 1.66; }
.body > ul, .body > ol { margin-bottom: 1.2em; }

/* Inline code sits inside prose, so it must not out-shout it. */
.body :not(pre) > code {
  font-size: .875em; padding: 1.5px 5px; border-radius: 5px;
  background: var(--surface-2); border: 1px solid var(--border-soft);
  color: var(--ink); white-space: nowrap;
}

.deflist { gap: 11px 24px; margin: var(--sp-3) 0 var(--sp-4); }
.deflist dt { font-size: 13px; }
.deflist dd { font-size: 15.2px; }

/* ========================================================== code ======== */

.codeblock {
  margin: var(--sp-3) 0 var(--sp-4);
  border-radius: 13px;
  border-color: var(--border);
  box-shadow: var(--e1);
}
.cb-head {
  padding: 9px 13px; gap: 10px;
  background: rgba(255,255,255,.026);
  border-bottom: 1px solid rgba(255,255,255,.07);
}
.cb-head::before {
  /* Three dots: the smallest possible signal that this is a file, not a
     quote. Purely conventional, and it costs nothing. */
  content: ""; flex: none; width: 34px; height: 8px;
  background:
    radial-gradient(circle 4px at 4px 4px,  rgba(255,255,255,.16) 98%, transparent) 0 0/12px 8px no-repeat,
    radial-gradient(circle 4px at 4px 4px,  rgba(255,255,255,.13) 98%, transparent) 12px 0/12px 8px no-repeat,
    radial-gradient(circle 4px at 4px 4px,  rgba(255,255,255,.10) 98%, transparent) 24px 0/12px 8px no-repeat;
}
.cb-title { font-size: 11.8px; color: #a6b1c2; }
.cb-copy { transition: color .15s var(--ease), border-color .15s var(--ease), background-color .15s var(--ease); }
.cb-copy:hover { background: rgba(255,255,255,.06); }
.codeblock pre { font-size: 13.3px; line-height: 1.66; padding: 15px 0; }
.cl { padding: 0 18px; }
.codeblock.numbered .cl { padding-left: 58px; }
.cb-caption { font-size: 13.6px; line-height: 1.6; margin: -12px 0 var(--sp-4); color: var(--ink-3); }

.output { border-radius: 12px; margin: 8px 0 var(--sp-4); }
.output-h { padding: 10px 15px 0; }
.output pre { padding: 6px 15px 14px; font-size: 12.7px; }

/* ======================================================= callouts ======= */

.callout {
  border-radius: 14px;
  margin: var(--sp-4) 0;
  border: 1px solid var(--border);
  background:
    linear-gradient(var(--c) 0 0) left / 3px 100% no-repeat,
    var(--surface);
  padding-left: 3px;
  box-shadow: var(--e1);
  overflow: hidden;
}
.callout-h {
  display: flex; align-items: center; gap: 10px;
  padding: 13px 18px 0 18px;
}
.callout-ic {
  display: inline-grid; place-items: center; flex: none;
  width: 26px; height: 26px; border-radius: 8px;
  background: var(--c-soft); border: 1px solid var(--c-line); color: var(--c);
}
.callout-ic svg { width: 14px; height: 14px; }
.callout-kind {
  font-size: 10.6px; font-weight: 700; letter-spacing: .11em;
  text-transform: uppercase; color: var(--c);
}
.callout-title { font-size: 15.4px; font-weight: 650; color: var(--ink); }
.callout-body { padding: 11px 18px 16px; }
.callout-body > *:last-child { margin-bottom: 0; }
.callout-body > p { font-size: 15.2px; line-height: 1.66; max-width: 66ch; }
.callout-body .codeblock { margin: 13px 0 var(--sp-3); }

/* The scenario callout closes a lesson, so it earns a heavier presence. */
.callout.scenario { background: linear-gradient(var(--c) 0 0) left / 3px 100% no-repeat, var(--surface-2); }

/* ========================================================= ladder ======= */

.ladder { margin: var(--sp-4) 0 var(--sp-5); }
.ladder-h {
  display: flex; align-items: center; gap: 10px;
  font-size: 10.6px; font-weight: 700; letter-spacing: .12em;
  text-transform: uppercase; color: var(--ink-3); margin-bottom: 14px;
}
.ladder-h::after { content: ""; flex: 1 1 auto; height: 1px; background: var(--border); }

.rung {
  position: relative;
  border: 1px solid var(--border); border-radius: 14px;
  background: var(--surface); margin-bottom: 12px; overflow: hidden;
  box-shadow: var(--e1);
}
.rung:last-child { margin-bottom: 0; }
.rung.bad  { --c: var(--crit); }
.rung.ok   { --c: var(--warn); }
.rung.best { --c: var(--good); }
.rung { background: linear-gradient(var(--c) 0 0) left / 3px 100% no-repeat, var(--surface); }

.rung-h { display: flex; align-items: center; gap: 11px; padding: 13px 16px 12px 18px; }
.rung-badge {
  display: inline-flex; align-items: center; gap: 6px; flex: none;
  padding: 4px 10px; border-radius: 999px;
  background: var(--c-soft, color-mix(in srgb, var(--c) 12%, transparent));
  border: 1px solid color-mix(in srgb, var(--c) 36%, transparent);
  color: var(--c); font-size: 10.4px; font-weight: 700;
  letter-spacing: .1em; text-transform: uppercase;
}
.rung-label { font-size: 15px; font-weight: 640; color: var(--ink); }
.rung-why {
  padding: 0 18px 12px; font-size: 14.4px; line-height: 1.62;
  color: var(--ink-2); max-width: 74ch;
}
.rung .codeblock { margin: 0 12px 12px; border-radius: 11px; }
.rung-note {
  margin: 0 12px 12px; padding: 11px 14px; border-radius: 10px;
  background: var(--surface-2); border: 1px solid var(--border-soft);
  font-size: 13.6px; line-height: 1.6; color: var(--ink-2);
}

/* ======================================================= exercise ======= */

.exercise {
  margin: var(--sp-5) 0;
  border: 1px solid var(--accent-line); border-radius: 16px;
  background: var(--surface); overflow: hidden;
  box-shadow: var(--e2);
}
.ex-head {
  padding: 16px 20px 14px;
  background: var(--accent-soft);
  border-bottom: 1px solid var(--accent-line);
}
.ex-kind {
  font-size: 10.4px; font-weight: 700; letter-spacing: .12em;
  text-transform: uppercase; color: var(--accent-ink);
}
.ex-title { font-size: 19px; font-weight: 680; letter-spacing: -.012em; margin: 6px 0 10px; }
.ex-meta { display: flex; flex-wrap: wrap; gap: 8px; }
.ex-meta span {
  display: inline-flex; align-items: center; gap: 6px;
  padding: 4px 9px; border-radius: 7px; font-size: 11.8px;
  background: var(--bg-elev); border: 1px solid var(--border); color: var(--ink-3);
}
.ex-body { padding: 18px 20px 4px; }
.ex-req { margin: var(--sp-3) 0; padding: 15px 18px; border-radius: 12px;
  background: var(--surface-2); border: 1px solid var(--border); }
.ex-req-h {
  font-size: 10.4px; font-weight: 700; letter-spacing: .12em;
  text-transform: uppercase; color: var(--ink-3); margin-bottom: 11px;
}
.ex-req ol, .ex-req ul { margin: 0; padding: 0; list-style: none; counter-reset: r; }
.ex-req li {
  counter-increment: r; position: relative; padding-left: 28px;
  margin-bottom: 9px; font-size: 14.4px; line-height: 1.58; color: var(--ink-2);
}
.ex-req li:last-child { margin-bottom: 0; }
.ex-req li::before {
  content: counter(r); position: absolute; left: 0; top: 1px;
  width: 19px; height: 19px; border-radius: 6px;
  display: grid; place-items: center;
  font-family: var(--mono); font-size: 10px; font-weight: 700;
  background: var(--bg-elev); border: 1px solid var(--border); color: var(--ink-3);
}

/* ======================================================= disclosure ===== */

details.disclose, details.solution {
  margin: var(--sp-3) 0; border: 1px solid var(--border);
  border-radius: 12px; background: var(--surface-2); overflow: hidden;
}
details.solution { margin: var(--sp-3) 20px 20px; border-color: var(--good-line); }
details > summary {
  cursor: pointer; list-style: none; user-select: none;
  display: flex; align-items: center; gap: 10px;
  padding: 13px 16px; font-size: 14px; font-weight: 620; color: var(--ink-2);
  transition: color .15s var(--ease), background-color .15s var(--ease);
}
details > summary::-webkit-details-marker { display: none; }
details > summary:hover { color: var(--ink); background: var(--surface-3); }
details[open] > summary { border-bottom: 1px solid var(--border); color: var(--ink); }
details > summary::before {
  content: ""; flex: none; width: 8px; height: 8px;
  border-right: 1.8px solid currentColor; border-bottom: 1.8px solid currentColor;
  transform: rotate(-45deg) translate(-1px, -1px);
  transition: transform .18s var(--ease);
}
details[open] > summary::before { transform: rotate(45deg) translate(-1px, -1px); }
details.solution > summary { color: var(--good); }

/* ============================================================ quiz ====== */

.quiz { margin: var(--sp-6) 0 var(--sp-5); }
.quiz-h, .iv-h {
  display: flex; align-items: baseline; gap: 12px;
  padding-bottom: 12px; margin-bottom: var(--sp-3);
  border-bottom: 1px solid var(--border);
}
.quiz-h h2, .iv-h h2 { margin: 0; font-size: 21px; letter-spacing: -.014em; border: 0; padding: 0; }
.quiz-h .sub, .iv-h .sub { font-size: 13px; color: var(--ink-3); }

.q {
  border: 1px solid var(--border); border-radius: 14px;
  background: var(--surface); padding: 17px 19px; margin-bottom: 12px;
}
.q-stem { font-size: 15.4px; line-height: 1.6; font-weight: 600; margin-bottom: 13px; }
.q-stem .qn {
  font-family: var(--mono); font-size: 11px; color: var(--ink-3);
  margin-right: 9px; font-weight: 600;
}
.q-opts { display: grid; gap: 7px; }
.q-opt {
  display: flex; align-items: flex-start; gap: 11px; width: 100%; text-align: left;
  padding: 11px 13px; border-radius: 10px; cursor: pointer;
  background: var(--surface-2); border: 1px solid var(--border);
  font-size: 14.4px; line-height: 1.55; color: var(--ink-2);
  transition: border-color .14s var(--ease), background-color .14s var(--ease), color .14s var(--ease);
}
.q-opt:hover:not(.picked) { border-color: var(--border-strong); color: var(--ink); }
.q-opt .k {
  flex: none; width: 21px; height: 21px; border-radius: 6px;
  display: grid; place-items: center;
  font-family: var(--mono); font-size: 10.5px; font-weight: 700;
  background: var(--bg-elev); border: 1px solid var(--border); color: var(--ink-3);
  transition: inherit;
}
.q-opt.right { border-color: var(--good-line); background: var(--good-soft); color: var(--ink); }
.q-opt.right .k { background: var(--good); border-color: var(--good); color: #06231a; }
.q-opt.wrong { border-color: var(--crit-line); background: var(--crit-soft); color: var(--ink); }
.q-opt.wrong .k { background: var(--crit); border-color: var(--crit); color: #2a0710; }
.q-why {
  margin-top: 12px; padding: 13px 15px; border-radius: 10px;
  background: var(--surface-2); border: 1px solid var(--border);
  border-left: 3px solid var(--accent);
  font-size: 14.2px; line-height: 1.62; color: var(--ink-2);
}
.q-why b, .q-why strong { color: var(--ink); }

/* ====================================================== interview ======= */

.iv { margin: var(--sp-5) 0; }
.iv-q {
  border: 1px solid var(--border); border-radius: 14px;
  background: var(--surface); margin-bottom: 12px; overflow: hidden;
}
.iv-q > summary { padding: 15px 18px; font-size: 15.2px; font-weight: 620; color: var(--ink); }
.iv-level {
  flex: none; padding: 3px 8px; border-radius: 999px;
  font-size: 9.8px; font-weight: 700; letter-spacing: .1em; text-transform: uppercase;
  border: 1px solid var(--border); background: var(--surface-2); color: var(--ink-3);
}
.iv-strong {
  margin: 0; padding: 14px 18px;
  background: var(--good-soft); border-bottom: 1px solid var(--border);
  font-size: 14.6px; line-height: 1.62; color: var(--ink);
}
.iv-strong::before {
  content: "A strong answer"; display: block;
  font-size: 10.2px; font-weight: 700; letter-spacing: .12em;
  text-transform: uppercase; color: var(--good); margin-bottom: 7px;
}
.iv-body { padding: 15px 18px; }
.iv-body > *:last-child { margin-bottom: 0; }
.iv-body > p { font-size: 14.8px; line-height: 1.66; color: var(--ink-2); max-width: 68ch; }

/* ======================================================= takeaways ====== */

.takeaways {
  margin: var(--sp-6) 0 var(--sp-5); padding: 22px 24px;
  border: 1px solid var(--border); border-radius: 16px;
  background: var(--surface-2);
}
.takeaways h2 { margin: 0 0 16px; font-size: 20px; border: 0; padding: 0; letter-spacing: -.014em; }
.takeaways ol { counter-reset: t; list-style: none; margin: 0; padding: 0; }
.takeaways li {
  counter-increment: t; position: relative; padding-left: 32px;
  margin-bottom: 11px; font-size: 15.1px; line-height: 1.62; color: var(--ink-2);
  max-width: 74ch;
}
.takeaways li:last-child { margin-bottom: 0; }
.takeaways li::before {
  content: counter(t, decimal-leading-zero); position: absolute; left: 0; top: 2px;
  font-family: var(--mono); font-size: 10.6px; font-weight: 700; color: var(--accent-ink);
}
.takeaways li strong { color: var(--ink); }

/* ========================================================== tables ====== */

.tbl-wrap { margin: var(--sp-3) 0 var(--sp-4); border: 1px solid var(--border);
  border-radius: 12px; overflow: hidden; }
.tbl-wrap table { margin: 0; border: 0; border-radius: 0; }
.body table th {
  font-size: 11.4px; font-weight: 700; letter-spacing: .06em; text-transform: uppercase;
  color: var(--ink-3); background: var(--surface-2); padding: 11px 14px;
}
.body table td { font-size: 14.2px; line-height: 1.58; padding: 11px 14px; vertical-align: top; }
.body table tr:hover td { background: color-mix(in srgb, var(--accent) 3.5%, transparent); }

/* ============================================================= toc ====== */

.toc-h {
  font-size: 10.4px; font-weight: 700; letter-spacing: .12em; text-transform: uppercase;
  color: var(--ink-3); padding-bottom: 11px; margin-bottom: 6px;
  border-bottom: 1px solid var(--border-soft);
}
.toc ul { list-style: none; margin: 0; padding: 0; border-left: 1px solid var(--border); }
.toc li a {
  display: block; position: relative; padding: 6px 0 6px 15px; margin-left: -1px;
  border-left: 2px solid transparent;
  font-size: 12.9px; line-height: 1.45; color: var(--ink-3); text-decoration: none;
  transition: color .14s var(--ease), border-color .14s var(--ease);
}
.toc li.sub a { padding-left: 26px; font-size: 12.3px; }
.toc li a:hover { color: var(--ink-2); border-left-color: var(--border-strong); text-decoration: none; }
.toc li a.on { color: var(--accent-ink); border-left-color: var(--accent); font-weight: 600; }

/* A problem set lists its drills instead of headings. Questions are a
   sentence long, so they clamp to two lines — the number stays readable and
   the panel keeps its width. */
.toc li.drill-item a {
  font-size: 12.1px; line-height: 1.38; padding-top: 5px; padding-bottom: 5px;
  display: -webkit-box; -webkit-line-clamp: 2; -webkit-box-orient: vertical;
  overflow: hidden;
}

/* ============================================================ rail ====== */

.rail-course-name { font-size: 14.6px; font-weight: 680; letter-spacing: -.01em; }
.rail-course-meta { font-size: 11.6px; color: var(--ink-3); }
.rail-phase {
  font-size: 9.8px; font-weight: 700; letter-spacing: .13em; text-transform: uppercase;
  color: var(--ink-3); padding: 18px 4px 8px; opacity: .82;
}
.mod-btn {
  border-radius: 9px; padding: 8px 9px; gap: 9px;
  transition: background-color .14s var(--ease), color .14s var(--ease);
}
.mod-btn:hover { background: var(--surface-2); }
.mod.active > .mod-btn { background: var(--surface-2); }
.mod-lv {
  font-family: var(--mono); font-size: 9.6px; font-weight: 700;
  padding: 3px 5px; border-radius: 5px;
  background: var(--surface-3); border: 1px solid var(--border); color: var(--ink-3);
}
.mod.active > .mod-btn .mod-lv { background: var(--accent-soft); border-color: var(--accent-line); color: var(--accent-ink); }
.mod-name { font-size: 13.1px; font-weight: 600; }
.mod-count { font-size: 10.6px; color: var(--ink-3); font-family: var(--mono); }
.mod-chev { transition: transform .18s var(--ease); }

.lsn a {
  display: flex; align-items: baseline; gap: 9px; padding: 6px 9px 6px 12px;
  border-radius: 8px; border-left: 2px solid transparent; margin-left: 11px;
  font-size: 12.9px; line-height: 1.45; color: var(--ink-3); text-decoration: none;
  transition: color .14s var(--ease), background-color .14s var(--ease), border-color .14s var(--ease);
}
.lsn a:hover { color: var(--ink-2); background: var(--surface-2); text-decoration: none; }
.lsn.cur a {
  color: var(--ink); background: var(--accent-soft);
  border-left-color: var(--accent); font-weight: 600;
}
.lsn-n { font-family: var(--mono); font-size: 10.4px; color: var(--ink-3); flex: none; }
.lsn.cur .lsn-n { color: var(--accent-ink); }
.lsn.locked a { opacity: .46; cursor: default; }
.lsn-soon {
  margin-left: auto; font-size: 9.2px; letter-spacing: .08em; text-transform: uppercase;
  color: var(--ink-3); border: 1px solid var(--border); border-radius: 4px; padding: 1px 4px;
}

/* ========================================================== pager ======= */

.pager {
  display: grid; grid-template-columns: 1fr 1fr; gap: 12px;
  margin: var(--sp-6) 0 var(--sp-7);
  padding-top: var(--sp-4); border-top: 1px solid var(--border);
}
.pager a {
  display: flex; flex-direction: column; gap: 7px;
  padding: 16px 18px; border-radius: 14px;
  background: var(--surface); border: 1px solid var(--border);
  text-decoration: none; min-height: 74px;
  transition: border-color .16s var(--ease), background-color .16s var(--ease), transform .16s var(--ease);
}
.pager a:not(.ph):hover {
  border-color: var(--accent-line); background: var(--surface-2);
  text-decoration: none; transform: translateY(-1px);
}
.pager a.nx { align-items: flex-end; text-align: right; }
.pager .dir {
  display: inline-flex; align-items: center; gap: 6px;
  font-size: 10.4px; font-weight: 700; letter-spacing: .12em;
  text-transform: uppercase; color: var(--ink-3);
}
.pager .dir svg { width: 13px; height: 13px; }
.pager a:not(.ph):hover .dir { color: var(--accent-ink); }
.pager .t { font-size: 14.8px; font-weight: 620; color: var(--ink); line-height: 1.4; }
.pager .ph { opacity: .46; cursor: default; }
.pager .ph .t { color: var(--ink-3); font-weight: 500; }

/* ========================================================= mobile ======= */

@media (max-width: 1180px) {
  .toc { display: none; }
}
@media (max-width: 860px) {
  .lh h1 { font-size: 28px; max-width: none; }
  .lh-lede { font-size: 16.4px; }
  .lh-contract { grid-template-columns: 1fr; }
  .body > p { font-size: 15.8px; }
  .body > h2 { margin-top: var(--sp-6); padding-top: var(--sp-4); font-size: 20px; gap: 10px; }
  .pager { grid-template-columns: 1fr; }
  .pager a.nx { align-items: flex-start; text-align: left; }
  .exercise { border-radius: 13px; }
  .ex-head, .ex-body { padding-left: 15px; padding-right: 15px; }
  details.solution { margin-left: 15px; margin-right: 15px; }
  .takeaways { padding: 18px 16px; }
}

/* ========================================================== print ======= */

@media print {
  .topbar, .rail, .toc, .pager, .reading-bar, .cb-copy { display: none !important; }
  .shell { display: block; }
  details { open: true; }
  .callout, .rung, .exercise, .q { break-inside: avoid; }
}

/* ====================================================== track tabs ====== */
/* On the course landing page the two tracks are peers, so they get equal
   visual weight and a tab each — not a primary path with an afterthought. */
.track-tabs {
  display: flex; gap: 10px; margin: 0 0 var(--sp-4); flex-wrap: wrap;
}
.ttab {
  flex: 1 1 260px; text-align: left; cursor: pointer;
  display: flex; flex-direction: column; gap: 4px;
  padding: 14px 17px; border-radius: 13px;
  background: var(--surface); border: 1px solid var(--border); color: var(--ink-3);
  font: inherit;
  transition: border-color .16s var(--ease), background-color .16s var(--ease), color .16s var(--ease);
}
.ttab:hover { border-color: var(--border-strong); color: var(--ink-2); }
.ttab.on {
  color: var(--ink); border-color: var(--accent-line);
  background: linear-gradient(var(--accent) 0 0) left / 3px 100% no-repeat, var(--accent-soft);
  padding-left: 20px;
}
.ttab-t { font-size: 16px; font-weight: 660; color: var(--ink); letter-spacing: -.012em; }
.ttab-m { font-size: 12.4px; color: var(--ink-3); }
.ttab.on .ttab-m { color: var(--accent-ink); }

/* =========================================================== drills ===== */
/* A practice page carries hundreds of these, so the resting state has to be
   quiet and scannable: one line per question, the answer folded away. */

.drill {
  border: 1px solid var(--border); border-radius: 11px;
  background: var(--surface); margin: 0 0 8px; overflow: hidden;
  transition: border-color .15s var(--ease);
}
.drill:hover { border-color: var(--border-strong); }
.drill[open] { border-color: var(--accent-line); background: var(--surface-2); }

.drill > summary {
  display: flex; align-items: baseline; gap: 12px;
  padding: 12px 15px; cursor: pointer; list-style: none; user-select: none;
  font-size: 14.8px; line-height: 1.5; font-weight: 600; color: var(--ink-2);
  transition: color .15s var(--ease);
}
.drill > summary::-webkit-details-marker { display: none; }
.drill > summary::before { content: none; }
.drill > summary:hover { color: var(--ink); }
.drill[open] > summary { color: var(--ink); border-bottom: 1px solid var(--border); }

.drill-n {
  flex: none; min-width: 30px;
  font-family: var(--mono); font-size: 10.6px; font-weight: 700;
  color: var(--ink-3); padding-top: 2px;
}
.drill[open] .drill-n { color: var(--accent-ink); }
.drill-q { flex: 1 1 auto; }
.drill-q code { font-size: .86em; }

/* The cue is the affordance: without it a reader does not know the answer
   is there at all. It disappears once opened, because then it is obvious. */
.drill-cue {
  flex: none; align-self: center;
  font-size: 10px; font-weight: 700; letter-spacing: .1em; text-transform: uppercase;
  color: var(--ink-3); border: 1px solid var(--border); border-radius: 999px;
  padding: 3px 9px; background: var(--bg-elev);
  transition: color .15s var(--ease), border-color .15s var(--ease);
}
.drill > summary:hover .drill-cue { color: var(--accent-ink); border-color: var(--accent-line); }
.drill[open] .drill-cue { display: none; }

.drill-a { padding: 14px 15px 4px; }
.drill-a > *:last-child { margin-bottom: 12px; }
.drill-a > p { font-size: 14.8px; line-height: 1.64; max-width: 70ch; }
.drill-a > ul, .drill-a > ol { font-size: 14.6px; max-width: 70ch; }
.drill-a .codeblock { margin: 10px 0 14px; }
.drill-a .codeblock:first-child { margin-top: 0; }
.drill-a h4 { margin: 16px 0 7px; font-size: 14.4px; color: var(--ink-2); }

.drill-terms { display: flex; flex-wrap: wrap; gap: 6px; margin-bottom: 12px; }
.kt {
  font-size: 10.8px; font-weight: 600; padding: 3px 8px; border-radius: 999px;
  background: var(--accent-soft); border: 1px solid var(--accent-line);
  color: var(--accent-ink);
}

/* Predict-the-output drills read differently from programs, so they carry a
   different accent — the question is the point, not the code. */
.drill.predict { background: linear-gradient(var(--warn) 0 0) left / 3px 100% no-repeat, var(--surface); }
.drill.predict[open] { background: linear-gradient(var(--warn) 0 0) left / 3px 100% no-repeat, var(--surface-2); }
.drill.predict .drill-n, .drill.predict[open] .drill-n { color: var(--warn); }

/* Bulk controls for a page of drills. */
.drill-bar {
  display: flex; align-items: center; gap: 10px; flex-wrap: wrap;
  margin: 0 0 var(--sp-3); padding: 11px 14px;
  border: 1px solid var(--border); border-radius: 11px; background: var(--surface-2);
  position: sticky; top: calc(var(--topbar) + 8px); z-index: 5;
  backdrop-filter: blur(10px);
}
.drill-bar .sp { flex: 1 1 auto; }
.drill-bar-c { font-size: 12.4px; color: var(--ink-3); }
.drill-bar-c b { color: var(--ink-2); font-weight: 640; }
.drill-btn {
  font: inherit; font-size: 12px; font-weight: 600; cursor: pointer;
  padding: 6px 12px; border-radius: 8px;
  background: var(--bg-elev); border: 1px solid var(--border); color: var(--ink-2);
  transition: color .15s var(--ease), border-color .15s var(--ease);
}
.drill-btn:hover { color: var(--ink); border-color: var(--border-strong); }
.drill-btn kbd {
  display: inline-block; margin-left: 6px; padding: 1px 5px;
  font-family: var(--mono); font-size: 9.6px; border-radius: 4px;
  background: var(--surface-3); border: 1px solid var(--border); color: var(--ink-3);
}

/* The rail's lesson list had a vertical rule marking the module's extent.
   The current lesson already carries an accent bar, so the rule was a second
   vertical line competing with it — removed, and the indent kept. */
.mod-list::before { content: none; }

/* The current lesson is marked by its filled box alone. The accent bar beside
   it was a second marker for the same state, and two markers read as two
   things. */
.lsn a { border-left: 0; margin-left: 9px; }
.lsn.cur a { border-left: 0; }

/* ======================================================= maths + banks ===== */
/* Display maths: KaTeX renders into this box; until it loads, or offline, the
   TeX itself shows in the mono face so the formula is still readable. */
.math { margin: 14px 0 20px; padding: 12px 18px; overflow-x: auto; border-radius: 10px;
  background: var(--surface-2); border: 1px solid var(--border-soft);
  font-family: var(--mono); font-size: 13px; color: var(--ink-2); text-align: center; }
.math .katex-display { margin: 0; }
.math .katex { font-size: 1.18em; color: var(--ink); }
.katex { font-size: 1.02em; }

/* Where a module's material comes from, shown on the card and in the lesson
   header so the mirror of the reference is visible, not just claimed. */
.mcard-src { display: inline-flex; align-items: center; gap: 6px; margin: 4px 0 6px;
  font-family: var(--mono); font-size: 10.6px; color: var(--ink-3); }
.mcard-src svg { width: 11px; height: 11px; opacity: .7; }
.mcard-outcome { display: flex; gap: 9px; align-items: flex-start; margin: 4px 0 12px;
  padding: 10px 12px; border-radius: 9px; background: var(--accent-soft);
  color: var(--ink-2); font-size: 13px; line-height: 1.5; }
.mcard-outcome svg { width: 15px; height: 15px; flex: none; margin-top: 2px; color: var(--accent-ink); }
.ttab-b { font-size: 12.2px; color: var(--ink-3); line-height: 1.45; margin-top: 2px; }

/* A program drill opens on code; give the summary the mono program number. */
.drill.program > summary .drill-n { color: var(--accent-ink); }