:root{color-scheme:light;--paper:#f6f5ef;--white:#fff;--ink:#203638;--muted:#52686b;--line:#c9d5d3;--teal:#166b63;--teal-soft:#eaf4f0;--blue:#2259a0;--amber:#a45611;--amber-soft:#fff3de;--radius:18px;font-family:system-ui,-apple-system,"Segoe UI",sans-serif;color:var(--ink);background:var(--paper)}
*{box-sizing:border-box}body{margin:0}button,select,input{font:inherit}button,select{min-height:44px;border:1px solid var(--line);border-radius:9px;background:white;color:var(--ink);padding:10px 15px;cursor:pointer}button:hover:not(:disabled){background:var(--teal-soft);border-color:var(--teal)}button:disabled{cursor:default;opacity:.43}button.primary{background:var(--teal);color:white;border-color:var(--teal);font-weight:700}button.primary:hover{background:#10594f}button:focus-visible,select:focus-visible,a:focus-visible,input:focus-visible,[tabindex]:focus-visible{outline:3px solid #a45611;outline-offset:4px}a{color:var(--teal);text-underline-offset:4px}p{margin:0;line-height:1.55}h1,h2,h3{margin:0;line-height:1.2}h1{font-size:clamp(25px,2.4vw,35px);letter-spacing:-.025em}h2{font-size:20px}h3{font-size:16px}.skip{position:fixed;top:-80px;left:15px;z-index:20;background:white;padding:14px}.skip:focus{top:10px}.masthead{max-width:1540px;padding:24px 36px 19px;margin:auto;display:flex;align-items:center;justify-content:space-between;gap:20px}.brand{display:flex;gap:15px;align-items:center}.brand svg{width:46px;flex-shrink:0;stroke:var(--teal);fill:var(--paper);stroke-width:2.2}.brand .filled{fill:var(--teal)}.brand strong{font-size:22px;display:block;letter-spacing:-.025em}.brand span{display:block;color:var(--muted);font-size:15px;margin-top:3px}.brand small{display:block;font-size:11px;letter-spacing:.1em;text-transform:uppercase;margin-top:7px;color:var(--teal)}.utilities{display:flex;gap:6px}.utilities button{border-color:transparent;background:transparent;font-size:13px;padding:9px 11px}.lesson-nav{display:flex;gap:9px;padding:0 36px 17px;border-bottom:1px solid var(--line);max-width:1540px;margin:auto}.lesson-nav button{background:transparent;border-color:transparent;display:flex;gap:13px;align-items:center;font-weight:550}.lesson-nav span{font-size:11px;color:var(--muted)}.lesson-nav button[aria-current]{background:var(--ink);color:white}.lesson-nav button[aria-current] span{color:#bad4ce}main{max-width:1540px;margin:auto;padding:25px 36px 22px}.intro{display:flex;align-items:center;justify-content:space-between;gap:35px;margin-bottom:18px}.intro #lead{font-size:15px;max-width:380px;color:var(--muted)}.eyebrow{text-transform:uppercase;letter-spacing:.11em;color:var(--teal);font-size:10.5px;font-weight:700;margin-bottom:8px}.choices{display:flex;gap:9px;align-items:center;margin-bottom:15px;flex-wrap:wrap}.choices button[aria-pressed=true]{background:var(--teal-soft);border-color:var(--teal);color:var(--teal)}.choices label{font-size:14px;font-weight:600}.choices select{max-width:100%}.choices:empty{display:none}.story{font-size:15px;background:#eeeee5;border-left:3px solid #aabdb5;border-radius:0 9px 9px 0;padding:13px 17px;margin-bottom:17px}.story b{font-weight:650}.model-inline{white-space:nowrap;font-family:Georgia,"Cambria Math",serif;font-size:20px;margin-right:14px}.story .subtle{font-size:13px;display:block;margin-top:3px}.workspace{display:grid;grid-template-columns:minmax(300px,.85fr) minmax(0,1.55fr);background:white;border:1px solid var(--line);border-radius:var(--radius);box-shadow:0 13px 36px #29352d08;overflow:hidden}.reasoning{padding:24px;border-right:1px solid var(--line);min-width:0}.reasoning h2{margin-bottom:9px}.reasoning .condition-words{color:var(--teal);font-weight:600;font-size:16px;margin-bottom:14px}.start-card{background:var(--teal-soft);border-radius:12px;padding:17px;margin:16px 0}.start-card h3{color:var(--teal);margin-bottom:9px}.start-card p{font-size:15px}.start-card strong{white-space:normal}.subtle{font-size:13px;color:var(--muted)}.reason-label{font-size:10px;color:var(--muted);letter-spacing:.1em;text-transform:uppercase;font-weight:650;margin-bottom:8px}.algebra-block{padding:11px 0;border-top:1px solid #e4ebe7}.algebra-block:first-child{border-top:0;padding-top:0}.math{font-family:"Cambria Math",Cambria,Georgia,serif;font-size:27px;line-height:1.6;color:var(--ink)}.math.compact{font-size:21px}.math.result{font-size:29px;color:var(--blue);font-weight:500}.equation-row{white-space:nowrap;overflow-x:auto;padding:2px 0 3px}.math i{font-family:Cambria,Georgia,serif}.frac{display:inline-flex;vertical-align:middle;flex-direction:column;align-items:center;margin:0 3px;line-height:1.23}.frac>span:first-child{border-bottom:1px solid currentColor;padding:0 4px 3px}.frac>span:last-child{padding-top:3px}.operation{color:var(--teal)}.reason{font-size:13px;color:var(--muted);margin:5px 0 0}.branch{border-left:3px solid #a4b5cb;padding-left:12px;margin:10px 0 14px}.branch h3{font-size:13px;color:var(--muted);margin-bottom:5px}.result-box{background:#edf3fb;border-radius:9px;padding:10px 13px;margin-top:12px}.result-box .math{font-size:24px}.geometry{padding:22px 22px 17px;min-width:0}.graph-heading{display:flex;justify-content:space-between;align-items:start;gap:16px}.graph-heading p{font-size:13px;color:var(--muted);margin-top:5px}.graph-badge{white-space:nowrap;font-size:10px;letter-spacing:.05em;border:1px solid #d4dfd9;border-radius:20px;padding:5px 8px;color:var(--teal)}.graph-scroll{overflow-x:auto;overscroll-behavior-inline:contain;margin-top:10px;border-radius:6px}.graph-scroll svg{width:100%;height:auto;display:block;min-width:550px}.graph-scroll text{font-family:system-ui,-apple-system,"Segoe UI",sans-serif;fill:var(--ink);font-size:15px}.graph-scroll .axis-text{font-size:14px;fill:var(--muted)}.graph-scroll .section-label{font-size:15px;font-weight:650}.graph-scroll .point-text{font-size:16px;font-weight:650;paint-order:stroke;stroke:white;stroke-width:5px;stroke-linejoin:round}.grid-line{stroke:#e4eae6;stroke-width:.8}.axis-line{stroke:#8b9c99;stroke-width:1.2;fill:none}.faint-model{stroke:#a7b3b8;stroke-width:2.4;fill:none}.solution-path{stroke:var(--blue);stroke-width:4.5;fill:none;stroke-linecap:round}.solution-arrow{fill:var(--blue)}.solution-dot{stroke:var(--blue);stroke-width:2.3;fill:white}.solution-dot.closed{fill:var(--blue)}.guide{stroke:#9aa7b0;stroke-width:1;stroke-dasharray:3 5}.probe{stroke:var(--amber);stroke-width:3;fill:#fff4dc}.probe-guide{stroke:var(--amber);stroke-width:1.4;stroke-dasharray:4 4}.graph-caption{font-size:14px;line-height:1.5;min-height:44px;padding:11px 0 0;border-top:1px solid #e5ebe7}.legend-swatch{display:inline-block;width:24px;height:3px;background:var(--blue);vertical-align:middle;margin-right:6px}.scroll-hint{display:none;font-size:12px;color:var(--muted);padding-top:5px}.insight{display:flex;gap:14px;align-items:start;margin-top:16px;padding:13px 4px;font-size:16px}.insight:before{content:'↳';font-size:29px;line-height:1;color:var(--teal)}.explore{margin-top:17px;background:white;border:1px solid var(--line);border-radius:15px;padding:20px 24px;display:grid;grid-template-columns:minmax(230px,1fr) minmax(250px,1.3fr);gap:22px}.explore[hidden]{display:none}.explore h2{font-size:18px;margin-bottom:7px}.explore .time-label{margin-top:11px;display:flex;justify-content:space-between;font-size:14px;font-weight:600}.explore input{width:100%;accent-color:var(--amber);min-height:35px;cursor:pointer}.presets{display:flex;gap:7px;flex-wrap:wrap;margin-top:8px}.presets button{font-size:13px;padding:8px 10px;min-height:40px}.presets button[aria-pressed=true]{background:var(--amber-soft);border-color:var(--amber)}.probe-result{padding:13px 17px;background:#f9faf6;border-radius:9px;align-self:stretch}.probe-result .math{font-size:24px}.truth{font-size:14px;font-weight:650;color:var(--teal);margin-top:8px}.truth.no{color:#934824}.truths{display:flex;gap:8px;flex-wrap:wrap;margin-top:6px}.truths span{padding:4px 8px;border:1px solid #d4ddd3;border-radius:6px;font-size:13px}.controls{position:sticky;bottom:0;display:flex;align-items:center;justify-content:space-between;gap:20px;padding:15px 36px;background:#f6f5eff5;border-top:1px solid var(--line);backdrop-filter:blur(12px);z-index:5}.progress{min-width:200px}.progress>span{font-size:10px;letter-spacing:.09em;text-transform:uppercase;font-weight:700;color:var(--teal)}.progress p{font-size:12px;margin-top:5px;max-width:345px}.transport{display:flex;gap:9px}.transport .primary{min-width:200px}.motion{display:flex;gap:7px;align-items:center}.motion button,.motion select,.motion label{font-size:12px}.motion button{padding:8px 10px}.motion label{margin-left:5px}.folio{max-width:1540px;margin:auto;padding:14px 36px 20px;display:flex;justify-content:space-between;gap:15px;flex-wrap:wrap;font-size:11px;color:var(--muted)}.folio div{display:flex;gap:22px}.dialog-head{display:flex;gap:25px;justify-content:space-between;align-items:start;margin-bottom:15px}.dialog-head button{padding:0 12px;font-size:26px}.dialog-head h2{font-size:25px}dialog{color:var(--ink);background:var(--paper);border:1px solid var(--line);border-radius:18px;max-width:680px;width:calc(100% - 36px);max-height:85vh;padding:28px;box-shadow:0 25px 70px #0004}dialog::backdrop{background:#edf0eafa;backdrop-filter:blur(12px)}dialog p{margin:12px 0}dialog ol{padding-left:25px;line-height:1.6}dialog li{margin:9px 0}dialog .primary{margin-top:12px}.paper-location{font-size:12px;text-transform:uppercase;color:var(--teal);letter-spacing:.06em}.paper-condition{font-size:23px;font-weight:650;margin:15px 0}.math-key{margin-top:11px;font-size:13px;color:var(--muted)}
@media(min-width:1540px){.controls{padding-inline:max(36px,calc((100vw - 1468px)/2))}}
@media(max-width:1100px){.masthead,main{padding-inline:24px}.lesson-nav{padding-inline:24px}.workspace{grid-template-columns:minmax(270px,.9fr) minmax(0,1.4fr)}.reasoning{padding:20px}.geometry{padding:20px 14px}.graph-heading{display:block}.graph-badge{display:none}.controls{padding:12px 24px;gap:12px;flex-wrap:wrap}.progress{flex:1}.progress p{max-width:none}.motion{margin-left:auto}.intro #lead{max-width:300px}.scroll-hint{display:block}.folio{padding-inline:24px}}
@media(max-width:850px){.masthead{align-items:start}.brand strong{font-size:20px}.brand svg{width:35px}.utilities{flex-wrap:wrap;justify-content:end;max-width:155px}.utilities button{font-size:12px;min-height:35px;padding:6px 9px}.lesson-nav{gap:3px}.lesson-nav button{padding:9px 11px;font-size:14px;gap:8px}.intro{align-items:start;gap:20px}.intro #lead{max-width:260px}.workspace{grid-template-columns:1fr}.reasoning{border-right:0;border-bottom:1px solid var(--line)}.reasoning .start-card{max-width:600px}.algebra-block{display:block}.geometry{padding:21px}.graph-scroll svg{min-width:550px}.graph-heading{display:flex}.graph-badge{display:block}.scroll-hint{display:none}.controls .progress{min-width:180px}.transport .primary{min-width:180px}.motion{width:100%;justify-content:end}.explore{grid-template-columns:1fr 1fr}}
@media(max-width:540px){.masthead{padding:19px 16px 12px;display:block}.brand{gap:10px}.brand strong{font-size:20px}.utilities{margin-top:13px;max-width:none;justify-content:start;gap:9px}.utilities button{min-height:40px;padding:9px 11px;background:#eeeee6}.lesson-nav{padding:0 12px 13px;display:grid;grid-template-columns:1fr 1fr;gap:5px}.lesson-nav button{font-size:13px;text-align:left}.intro{display:block}.intro #lead{max-width:none;margin-top:10px;font-size:14px}main{padding:20px 16px 10px}.choices{gap:7px}.choices button{font-size:13px;padding:10px}.choices label{display:block}.choices select{font-size:12px;width:100%;padding:10px 7px}.story{font-size:14px;padding:12px}.reasoning{padding:20px 17px}.geometry{padding:18px 11px}.graph-heading h2{font-size:17px}.graph-badge{display:none}.scroll-hint{display:block}.graph-scroll{margin-left:-4px;margin-right:-4px}.explore{grid-template-columns:1fr;padding:18px}.controls{position:relative;padding:14px 16px;gap:13px}.progress{width:100%;flex:auto}.transport{width:100%}.transport .primary{flex:1;min-width:0}.motion{justify-content:start;flex-wrap:wrap}.motion label{margin-left:auto}.motion select{max-width:130px}.folio{padding:14px 16px 20px;display:block}.folio div{margin-top:12px;flex-direction:column;gap:14px}.insight{font-size:14px}.math{font-size:25px}.math.compact{font-size:19px}.result-box .math{font-size:23px}dialog{padding:21px}.dialog-head h2{font-size:22px}}
@media(prefers-reduced-motion:reduce){*{scroll-behavior:auto}}
.axis-fill{fill:#8b9c99}.model-fill{fill:#a7b3b8}.review-steps{margin-top:20px;border-top:1px solid var(--line);padding-top:13px}.review-steps summary{color:var(--teal);font-size:14px;font-weight:600;cursor:pointer;min-height:36px}.review-steps[open] summary{margin-bottom:12px}.review-steps .math{font-size:23px}.review-steps .math.compact{font-size:19px}