/* Layout idea: one dark marble-and-cyan world on a single 12-column grid. Every block starts on column 1 or column 7,
   so left and right edges line up from the header to the footer. Each section holds one piece of live mathematics. */
:root{
  color-scheme:dark;
  --ink:#0a1215;        /* brand night, sampled from the logo */
  --deep:#101d21;       /* raised panels */
  --marble:#e8eeed;     /* statue highlights, main text */
  --stone:#93a5a8;      /* secondary text */
  --cyan:#59e2df;       /* brand accent */
  --paper:#eef3f1;      /* trainer sheet */
  --paper-ink:#0f1b1e;
  --line:rgba(232,238,237,.14);
  --ok:#1c8a6b; --retry:#b4552f;
  --display:"Onest","Segoe UI",Arial,sans-serif;
  --body:"Onest","Segoe UI",Arial,sans-serif;
  --math:"Cormorant Garamond",Georgia,"Times New Roman",serif;
  --wrap:1240px;
  --gap:clamp(16px,2.4vw,32px);
  --sec:clamp(72px,9vw,128px);
}
*{box-sizing:border-box}
html{scroll-behavior:smooth;scroll-padding-top:84px}
body{margin:0;background:var(--ink);color:var(--marble);font:17px/1.55 var(--body);-webkit-font-smoothing:antialiased;overflow-x:hidden}
img,canvas{max-width:100%;display:block}
a{color:inherit;text-decoration:none}
button,input{font:inherit;color:inherit}
button{cursor:pointer;border:0;background:none;padding:0}
h1,h2,h3,p{margin:0}
h1,h2,h3{font-family:var(--display);font-weight:600;letter-spacing:-.035em;line-height:1.04;text-wrap:balance}
h2{font-size:clamp(32px,4.2vw,56px)}
h3{font-size:clamp(21px,2vw,26px);letter-spacing:-.025em;line-height:1.15}
:focus-visible{outline:2px solid var(--cyan);outline-offset:4px;border-radius:6px}
.vh{position:absolute;width:1px;height:1px;overflow:hidden;clip-path:inset(50%);white-space:nowrap}
.wrap{max-width:var(--wrap);margin:0 auto;padding:0 clamp(18px,4vw,48px)}
.grid{display:grid;grid-template-columns:repeat(12,minmax(0,1fr));column-gap:var(--gap);row-gap:clamp(28px,5vw,48px)}
.cL{grid-column:1/span 5}
.cR{grid-column:7/span 6}
.lead{color:var(--stone);max-width:44ch}
.math{font-family:var(--math);font-style:italic;font-weight:500}

.btn{display:inline-flex;align-items:center;justify-content:center;min-height:50px;padding:0 28px;border-radius:999px;background:var(--cyan);color:var(--ink);font-weight:600;font-size:15px;transition:transform .18s,box-shadow .18s}
.btn:hover{transform:translateY(-2px);box-shadow:0 10px 30px -8px rgba(89,226,223,.55)}
.btn.ghost{background:transparent;color:var(--marble);box-shadow:inset 0 0 0 1px var(--line)}
.btn.ghost:hover{box-shadow:inset 0 0 0 1px var(--cyan)}
.btn.inked{background:var(--paper-ink);color:#fff}
.btn.inked:hover{box-shadow:0 10px 26px -10px rgba(15,27,30,.7)}
.btn:disabled{opacity:.45;cursor:default;transform:none;box-shadow:none}

/* header */
.top{position:sticky;top:0;z-index:20;transition:background .25s,border-color .25s;border-bottom:1px solid transparent}
.top.stuck{background:var(--ink);border-color:var(--line)}
.top .grid{align-items:center;height:76px;row-gap:0}
.logo{grid-column:1/span 5;justify-self:start}
.logo img{height:46px;width:auto;mix-blend-mode:lighten}
.top nav{grid-column:7/span 4;position:relative;display:flex;gap:28px;font-size:15px;align-self:stretch;align-items:center}
.top nav a{color:var(--stone);transition:color .15s;white-space:nowrap}
.top nav a:hover,.top nav a.on{color:var(--marble)}
#nav-ind{position:absolute;left:0;bottom:-1px;height:2px;width:0;background:var(--cyan);transition:left .4s cubic-bezier(.3,.8,.2,1),width .4s cubic-bezier(.3,.8,.2,1)}
.top .btn{grid-column:11/span 2;justify-self:end;min-height:42px;padding:0 20px;font-size:14px;white-space:nowrap}

/* hero */
.hero{position:relative;margin-top:-77px;padding-top:77px;overflow:hidden;isolation:isolate;cursor:crosshair}
.hero a,.hero button{cursor:pointer}
.hero-art{position:absolute;inset:0 -4% 0 auto;width:min(66%,960px);z-index:-2;will-change:transform}
.hero-art img,.hero-art svg{position:absolute;inset:0;width:100%;height:100%;object-fit:cover;object-position:60% 20%}
.hero-art img{mix-blend-mode:lighten;-webkit-mask-image:linear-gradient(to right,transparent,#000 34%);mask-image:linear-gradient(to right,transparent,#000 34%)}
.hero-art path.draw{stroke-dasharray:1;stroke-dashoffset:0;animation:draw 2.2s cubic-bezier(.6,0,.2,1) .2s backwards}
@keyframes draw{from{stroke-dashoffset:1}}
#grid{position:absolute;inset:0;width:100%;height:100%;z-index:-1;pointer-events:none}
.tag{position:absolute;left:0;top:0;z-index:1;pointer-events:none;font:italic 500 19px/1 var(--math);color:var(--cyan);white-space:nowrap;text-shadow:0 0 12px var(--ink),0 0 4px var(--ink)}
#eq{font-size:26px;font-weight:600;color:var(--marble)}
.hero-main{padding:clamp(48px,8vw,104px) 0 clamp(44px,6vw,80px)}
.hero-text{grid-column:1/span 8}
.hero h1{font-size:clamp(42px,6.3vw,88px);line-height:.98;letter-spacing:-.045em}
.hero .grade{font-size:15px;color:var(--cyan);margin-bottom:22px}
.hero .intro{margin:26px 0 34px;font-size:clamp(17px,1.5vw,20px);color:#c6d2d3;max-width:34ch}
.cta-row{display:flex;flex-wrap:wrap;gap:12px}
.hero-hint{margin-top:26px;font-size:14px;color:var(--stone);max-width:44ch}
.hero-facts{list-style:none;margin:0;padding:0;border-top:1px solid var(--line);row-gap:0}
.hero-facts li{padding:22px 0 26px;font-size:15px;color:var(--stone)}
.hero-facts li:nth-child(1){grid-column:1/span 6}
.hero-facts li:nth-child(2){grid-column:7/span 3}
.hero-facts li:nth-child(3){grid-column:10/span 3}
.hero-facts b{display:block;font:italic 500 30px/1.15 var(--math);color:var(--marble);margin-bottom:4px;white-space:nowrap}

/* sine divider */
.wave{padding:clamp(40px,6vw,72px) 0 0}
.wave p{font:italic 500 22px/1 var(--math);color:var(--stone);font-variant-numeric:tabular-nums}
.wave p b{color:var(--cyan);font-weight:600}
#wave{width:100%;height:120px;margin-top:6px}

section{padding:var(--sec) 0}
.head{align-items:end;margin-bottom:clamp(32px,5vw,60px)}
.head h2{grid-column:1/span 6}

/* directions */
#directions{padding-top:clamp(36px,5vw,64px)}
.dirs{align-items:start}
.dir-list{display:flex;flex-direction:column}
.dir{position:relative;border-top:1px solid var(--line)}
.dir:last-child{border-bottom:1px solid var(--line)}
.dir::before{content:"";position:absolute;left:0;top:-1px;height:1px;width:100%;background:var(--cyan);transform:scaleX(0);transform-origin:0 50%;transition:transform .45s cubic-bezier(.2,.7,.2,1)}
.dir.on::before{transform:scaleX(1)}
.dir-pick{display:grid;grid-template-columns:1fr auto;gap:6px 20px;width:100%;text-align:left;padding:26px 0 10px}
.dir-pick h3{transition:color .2s}
.dir.on h3{color:var(--cyan)}
.dir-pick p{grid-column:1;color:var(--stone);font-size:16px;max-width:36ch}
.dir-pick .math{grid-row:1/3;grid-column:2;font-size:54px;line-height:1;color:var(--stone);align-self:center;transform-origin:100% 50%;transition:color .2s,transform .5s cubic-bezier(.2,.7,.2,1)}
.dir.on .math{color:var(--cyan);transform:scale(1.18) rotate(-6deg)}
.dir-more{display:inline-block;margin:0 0 24px;font-size:14px;font-weight:500;color:var(--stone);border-bottom:1px solid var(--line);padding-bottom:2px;transition:color .2s,border-color .2s}
.dir.on .dir-more{color:var(--marble);border-color:var(--cyan)}
.plot{position:sticky;top:104px;background:var(--deep);border-radius:22px;padding:22px 22px 20px;box-shadow:inset 0 0 0 1px var(--line)}
#plot{width:100%;aspect-ratio:5/4;touch-action:pan-y;cursor:crosshair}
.plot-read{min-height:2.7em;margin-top:14px;font:italic 500 23px/1.3 var(--math);color:var(--marble)}
.plot-read b{color:var(--cyan);font-weight:600;font-variant-numeric:tabular-nums}
.plot-tip{font-size:13px;color:var(--stone);margin-top:6px}

/* lessons: a number line */
#lessons{padding-top:0}
.lessons .cL p{margin-top:22px}
.steps{position:relative;list-style:none;margin:0;padding:0;--p:0;display:flex;flex-direction:column;gap:44px;
  background:repeating-linear-gradient(to bottom,var(--line) 0 1px,transparent 1px 14px) 22px 22px/7px calc(100% - 44px) no-repeat}
.steps::before,.steps::after{content:"";position:absolute;left:21px;top:22px;bottom:22px;width:1px;background:var(--line)}
.steps::after{background:var(--cyan);width:2px;left:20.5px;transform-origin:50% 0;transform:scaleY(var(--p))}
.steps li{position:relative;display:grid;grid-template-columns:44px 1fr;gap:24px}
.steps .n{position:relative;z-index:1;width:44px;height:44px;border-radius:50%;display:grid;place-items:center;background:var(--ink);box-shadow:inset 0 0 0 1px var(--line);font:italic 600 22px/1 var(--math);color:var(--stone);transition:color .3s,box-shadow .3s,background .3s,transform .6s cubic-bezier(.2,.7,.2,1)}
.steps li.reached .n{color:var(--ink);background:var(--cyan);box-shadow:0 0 0 6px rgba(89,226,223,.14);transform:rotate(360deg)}
.steps p{color:var(--stone);margin-top:8px;max-width:48ch}

/* trainer: the paper sheet, opened like a compass circle */
#trainer{padding:0}
.paper{background:var(--paper);color:var(--paper-ink);padding:clamp(56px,8vw,104px) 0;
  background-image:linear-gradient(rgba(15,27,30,.055) 1px,transparent 1px),linear-gradient(90deg,rgba(15,27,30,.055) 1px,transparent 1px);background-size:28px 28px;background-position:center top}
.paper .grid{align-items:start}
.paper :focus-visible{outline-color:var(--paper-ink)}
.paper .cL p{margin-top:18px;color:#42555a;max-width:30ch}
.paper .cL small{display:block;margin-top:22px;font-size:14px;color:#5c6f73;max-width:30ch}
.card{background:#fff;border-radius:20px;padding:clamp(18px,2.6vw,32px);box-shadow:0 30px 60px -40px rgba(15,27,30,.45),0 0 0 1px rgba(15,27,30,.08)}
.seg-label{font-size:13px;color:#5c6f73;margin-bottom:8px}
.seg{display:flex;flex-wrap:wrap;gap:6px;margin-bottom:18px}
.seg button{padding:9px 16px;border-radius:999px;font-size:14px;font-weight:500;box-shadow:inset 0 0 0 1px rgba(15,27,30,.16);transition:background .15s,color .15s}
.seg button:hover{background:rgba(15,27,30,.06)}
.seg button[aria-pressed="true"]{background:var(--paper-ink);color:#fff;box-shadow:none}
.q-meta{display:flex;justify-content:space-between;align-items:center;gap:12px;flex-wrap:wrap;margin:6px 0 16px;font-size:14px;color:#5c6f73;font-variant-numeric:tabular-nums}
.dots{display:flex;gap:8px}
.dots button{width:26px;height:26px;border-radius:50%;font-size:12px;font-weight:600;color:#5c6f73;box-shadow:inset 0 0 0 1px rgba(15,27,30,.2);transition:transform .15s}
.dots button:hover{transform:scale(1.12)}
.dots button.cur{box-shadow:inset 0 0 0 2px var(--paper-ink);color:var(--paper-ink)}
.dots button.done{background:var(--cyan);color:var(--paper-ink);box-shadow:none}
.dots button.done.cur{box-shadow:inset 0 0 0 2px var(--paper-ink)}
#quiz-question{display:block;font:500 clamp(20px,2.2vw,26px)/1.3 var(--display);letter-spacing:-.02em;min-height:3.9em;font-variant-numeric:tabular-nums}
.answer-row{display:flex;gap:10px;margin-top:18px;flex-wrap:wrap}
#quiz-answer{flex:1 1 180px;min-width:0;min-height:50px;padding:0 18px;border-radius:14px;border:1px solid rgba(15,27,30,.22);background:#fff;color:var(--paper-ink);font-size:18px;transition:border-color .15s,box-shadow .15s}
#quiz-answer:focus{outline:none;border-color:var(--paper-ink);box-shadow:0 0 0 3px rgba(89,226,223,.55)}
#quiz-answer.shake{animation:shake .35s}
@keyframes shake{25%{transform:translateX(-6px)}75%{transform:translateX(6px)}}
#quiz-format{display:block;margin-top:8px;font-size:13px;color:#5c6f73}
.feedback{min-height:1.6em;margin-top:12px;font-weight:600}
.feedback.ok{color:var(--ok)} .feedback.retry{color:var(--retry)}
.q-actions{display:flex;flex-wrap:wrap;gap:8px;align-items:center;margin-top:10px}
.chip{padding:9px 16px;border-radius:999px;font-size:14px;font-weight:500;box-shadow:inset 0 0 0 1px rgba(15,27,30,.2)}
.chip:hover{background:rgba(15,27,30,.06)}
.chip[aria-expanded="true"]{background:rgba(89,226,223,.35);box-shadow:none}
.q-actions .btn{min-height:40px;padding:0 20px;font-size:14px;margin-left:auto}
.help{display:grid;grid-template-rows:0fr;transition:grid-template-rows .3s}
.help.open{grid-template-rows:1fr}
.help>div{overflow:hidden}
.help p{margin-top:14px;padding:14px 16px;border-left:3px solid var(--cyan);background:#f3f8f7;border-radius:0 12px 12px 0;font-size:16px}
#fx{position:fixed;inset:0;width:100%;height:100%;pointer-events:none;z-index:30}

/* contact */
.contact{align-items:start}
.contact .lead{margin-top:22px}
#spiral{width:100%;aspect-ratio:1.618;margin-top:clamp(28px,4vw,44px)}
.phi{margin-top:10px;font:italic 500 19px/1.2 var(--math);color:var(--stone)}
.builder{background:var(--deep);border-radius:22px;padding:clamp(20px,3vw,34px);box-shadow:inset 0 0 0 1px var(--line)}
.builder .seg-label{color:var(--stone)}
.builder .seg button{box-shadow:inset 0 0 0 1px var(--line);color:var(--marble)}
.builder .seg button:hover{background:rgba(232,238,237,.07)}
.builder .seg button[aria-pressed="true"]{background:var(--cyan);color:var(--ink)}
.bubble{margin:8px 0 22px;padding:16px 18px;border-radius:18px 18px 18px 4px;background:rgba(89,226,223,.1);box-shadow:inset 0 0 0 1px rgba(89,226,223,.28);font-size:16px;min-height:4.6em}
.send{display:flex;flex-wrap:wrap;gap:14px 20px;align-items:center}
.send span{font-size:14px;color:var(--stone)}
.send b{color:var(--marble);font-weight:500}

footer{border-top:1px solid var(--line);padding:30px 0;font-size:14px;color:var(--stone)}
footer .grid{align-items:center;row-gap:14px}
footer .f-note{grid-column:7/span 4}
footer .f-cab{grid-column:11/span 2;justify-self:end;white-space:nowrap}
footer a:hover{color:var(--marble)}

@media (max-width:900px){
  .cL,.cR,.hero-text,.head h2,.hero-facts li:nth-child(n),footer .f-note{grid-column:1/-1}
  .top nav{display:none}
  .logo{grid-column:1/span 6}
  .top .btn{grid-column:7/span 6}
  footer .logo{grid-column:1/span 7}
  footer .f-cab{grid-column:8/span 5}
  .plot{position:static}
  .hero{cursor:auto}
  .hero-art{width:100%;inset:0;opacity:.3}
  .hero-art img{-webkit-mask-image:linear-gradient(to bottom,#000 40%,transparent);mask-image:linear-gradient(to bottom,#000 40%,transparent)}
  .tag,.hero-hint{display:none}
  .hero-facts li{padding:16px 0 18px}
  .hero-facts li+li{border-top:1px solid var(--line)}
}
@media (max-width:480px){
  .logo img{height:38px}
  .top .btn{padding:0 14px}
  .dir-pick .math{font-size:40px}
  .q-actions .btn{margin-left:0;width:100%}
  .hero-facts b{white-space:normal}
}
@media (prefers-reduced-motion:reduce){
  html{scroll-behavior:auto}
  *,*::before,*::after{animation:none!important;transition:none!important}
}

/* ---------- direction pages ---------- */
@view-transition{navigation:auto}
.sub .hero-main{align-items:center;padding:clamp(40px,6vw,84px) 0 clamp(40px,6vw,72px)}
.sub-text{grid-column:1/span 6}
.sub .hero h1,.sub h1{font-size:clamp(38px,5vw,68px);line-height:1}
.sub .plot{position:static}
.back{display:inline-flex;align-items:center;gap:10px;margin-bottom:26px;font-size:14px;color:var(--stone);transition:color .15s}
.back::before{content:"";width:22px;height:1px;background:currentColor;transition:width .25s}
.back:hover{color:var(--marble)} .back:hover::before{width:34px}
#topics{padding-top:clamp(36px,5vw,64px)}
.topics{row-gap:clamp(28px,4vw,44px)}
.topic{position:relative;grid-column:span 4;border-top:1px solid var(--line);padding-top:22px}
.topics.four .topic{grid-column:span 3}
.topic::before{content:"";position:absolute;left:0;top:-1px;height:1px;width:100%;background:var(--cyan);transform:scaleX(0);transform-origin:0 50%;transition:transform .45s cubic-bezier(.2,.7,.2,1)}
.topic:hover::before{transform:scaleX(1)}
.topic h3{transition:color .2s}
.topic:hover h3{color:var(--cyan)}
.topic p{margin-top:12px;color:var(--stone);font-size:16px}
#work{padding-top:0}
#work .cL p{margin-top:22px;color:var(--stone);max-width:44ch}
.source{display:inline-block;margin-top:22px;font-size:15px;font-weight:500;border-bottom:1px solid var(--cyan);padding-bottom:2px}
.work{list-style:none;margin:0;padding:0}
.work li{position:relative;padding:20px 0 20px 40px;border-top:1px solid var(--line);font-size:18px;line-height:1.45}
.work li:last-child{border-bottom:1px solid var(--line)}
.work li::before{content:"=";position:absolute;left:0;top:14px;font:italic 600 30px/1 var(--math);color:var(--cyan)}
.others{padding-top:0}
.others h2{font-size:clamp(24px,2.6vw,34px);margin-bottom:28px}
.other{grid-column:span 4;display:grid;grid-template-columns:1fr auto;gap:6px 16px;align-items:center;padding:22px 0;border-top:1px solid var(--line);border-bottom:1px solid var(--line);transition:border-color .25s}
.other:hover{border-color:var(--cyan)}
.other h3{font-size:21px;transition:color .2s}
.other:hover h3{color:var(--cyan)}
.other .math{font-size:40px;line-height:1;color:var(--stone);transition:color .2s,transform .5s cubic-bezier(.2,.7,.2,1)}
.other:hover .math{color:var(--cyan);transform:scale(1.15) rotate(-6deg)}
@media (max-width:900px){
  .sub-text,.topic,.topics.four .topic,.other{grid-column:1/-1}
  .other{border-bottom:0}
  .other:last-child{border-bottom:1px solid var(--line)}
  .others .grid{row-gap:0}
}