compare: selection UX on runs list + /compare placeholder page

- per-run checkbox when embedding HTML exists; cap at 2 selected
- sticky 'compare selected' button opens /compare?a=&b= in new tab
- selection state persists across the 3s htmx poll via a Set keyed by stem
- /compare stub validates stems, renders scaffolding (three.js UI next)

app/web/main.py

@@ -833,6 +833,18 @@ async def run_frames(stem: str) -> Response:
     return Response(content=payload, media_type="application/json")
 
 
+@app.get("/compare", response_class=HTMLResponse)
+async def compare_page(request: Request, a: str = "", b: str = "") -> HTMLResponse:
+    for label, stem in (("a", a), ("b", b)):
+        if not stem or not _STEM_RE.match(stem):
+            raise HTTPException(400, f"missing or malformed stem for {label!r}")
+        if not (FIGS_DIR / f"{stem}.html").is_file():
+            raise HTTPException(404, f"no such run: {stem}")
+    return templates.TemplateResponse(
+        request, "compare.html", {"stem_a": a, "stem_b": b}
+    )
+
+
 @app.get("/health")
 async def health() -> JSONResponse:
     async with httpx.AsyncClient(timeout=3.0) as client:

app/web/static/compare-select.js

@@ -0,0 +1,64 @@
+// Manages run-comparison selection on the runs list.
+// HTMX re-renders #runs-slot every 3s, so we keep state in a Set outside
+// the polled region and re-apply checked state on every afterSwap.
+
+(function () {
+  const MAX = 2;
+  const selected = new Set();
+
+  const btn = document.getElementById('compare-btn');
+  const countEl = document.getElementById('compare-count');
+  const slot = document.getElementById('runs-slot');
+  if (!btn || !countEl || !slot) return;
+
+  function refreshButton() {
+    const n = selected.size;
+    countEl.textContent = `(${n}/${MAX})`;
+    btn.disabled = n !== MAX;
+  }
+
+  function applyToDOM() {
+    const checkboxes = slot.querySelectorAll('.compare-cb');
+    // Drop any selected stems that are no longer in the DOM (run aged out of list)
+    const present = new Set();
+    checkboxes.forEach((cb) => present.add(cb.dataset.stem));
+    for (const s of [...selected]) if (!present.has(s)) selected.delete(s);
+
+    const atCap = selected.size >= MAX;
+    checkboxes.forEach((cb) => {
+      const stem = cb.dataset.stem;
+      cb.checked = selected.has(stem);
+      cb.disabled = atCap && !cb.checked;
+    });
+    refreshButton();
+  }
+
+  slot.addEventListener('change', (e) => {
+    const cb = e.target;
+    if (!cb.matches || !cb.matches('.compare-cb')) return;
+    const stem = cb.dataset.stem;
+    if (cb.checked) {
+      if (selected.size >= MAX) {
+        cb.checked = false;
+        return;
+      }
+      selected.add(stem);
+    } else {
+      selected.delete(stem);
+    }
+    applyToDOM();
+  });
+
+  document.body.addEventListener('htmx:afterSwap', (e) => {
+    if (e.target && e.target.id === 'runs-slot') applyToDOM();
+  });
+
+  btn.addEventListener('click', () => {
+    if (selected.size !== MAX) return;
+    const [a, b] = [...selected];
+    const url = `/compare?a=${encodeURIComponent(a)}&b=${encodeURIComponent(b)}`;
+    window.open(url, '_blank', 'noopener');
+  });
+
+  applyToDOM();
+})();

app/web/static/style.css

@@ -456,10 +456,63 @@ button.submit:disabled { background: var(--faint); border-color: var(--faint); c
   border-bottom: 1px solid var(--rule);
   padding: 0.85rem 0 0.85rem;
   display: grid;
-  grid-template-columns: 5.5rem 1fr;
-  column-gap: 1rem;
+  grid-template-columns: 1.25rem 5.5rem 1fr;
+  column-gap: 0.6rem;
   align-items: start;
 }
+.runs li.run > .compare-cb,
+.runs li.run > .compare-cb-slot {
+  width: 1rem;
+  height: 1rem;
+  margin: 3px 0 0 0;
+  align-self: start;
+  cursor: pointer;
+  accent-color: var(--accent);
+}
+.runs li.run > .compare-cb-slot {
+  visibility: hidden;
+  cursor: default;
+}
+.runs li.run > .compare-cb:disabled {
+  cursor: not-allowed;
+  opacity: 0.35;
+}
+
+.compare-bar {
+  display: flex;
+  align-items: center;
+  gap: 0.7rem;
+  margin-bottom: 0.5rem;
+  flex-wrap: wrap;
+}
+.compare-bar button {
+  font: inherit;
+  font-size: 0.82rem;
+  background: transparent;
+  border: 1px solid var(--rule);
+  color: var(--ink);
+  padding: 0.3rem 0.7rem;
+  cursor: pointer;
+  transition: background 100ms ease, border-color 100ms ease;
+}
+.compare-bar button:not(:disabled):hover {
+  background: var(--accent-tint);
+  border-color: var(--accent);
+}
+.compare-bar button:disabled {
+  opacity: 0.45;
+  cursor: not-allowed;
+}
+.compare-bar #compare-count {
+  font-family: var(--mono);
+  font-size: 0.76rem;
+  color: var(--faint);
+  margin-left: 0.25rem;
+}
+.compare-hint {
+  font-size: 0.76rem;
+  font-style: italic;
+}
 .runs li.run.just-submitted {
   background: linear-gradient(to right, var(--accent-tint), transparent 60%);
   padding-left: 0.55rem;

app/web/templates/_runs.html

@@ -8,6 +8,11 @@
   <ul class="runs">
     {% for r in runs %}
       <li class="run {% if just_submitted is defined and r.id == just_submitted %}just-submitted{% endif %}">
+        {% if r.emb_exists %}
+          <input type="checkbox" class="compare-cb" data-stem="{{ r.emb_file[:-5] }}" aria-label="select run for comparison" />
+        {% else %}
+          <span class="compare-cb-slot" aria-hidden="true"></span>
+        {% endif %}
         <div class="stamp">
           {% if r.runtime %}<span class="rt">{{ r.runtime }}</span>{% endif %}
           {% if r.start %}{{ r.start[:10] }}<br/>{{ r.start[11:19] }}{% else %}&nbsp;{% endif %}

app/web/templates/compare.html

@@ -0,0 +1,47 @@
+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8" />
+  <meta name="viewport" content="width=device-width,initial-scale=1" />
+  <title>embedding notebook &middot; compare</title>
+  <link rel="stylesheet" href="/static/style.css?v=20" />
+  <script>
+    (function(){try{
+      var t=localStorage.getItem('theme');
+      if(!t)t=matchMedia('(prefers-color-scheme: dark)').matches?'dark':'light';
+      document.documentElement.setAttribute('data-theme',t);
+    }catch(e){}})();
+  </script>
+</head>
+<body>
+
+<header class="masthead">
+  <div>
+    <h1 class="title">embedding notebook <em>&mdash; compare</em></h1>
+  </div>
+  <div class="meta">
+    <a href="/" class="masthead-link">&larr; runs</a>
+    <button type="button" class="theme-toggle" id="theme-toggle" aria-label="toggle theme">&#9680;</button>
+  </div>
+</header>
+
+<section style="padding: 2rem 1.2rem; max-width: 72ch; margin: 0 auto; font-family: var(--serif);">
+  <p style="color: var(--mute);">
+    Comparison page &mdash; scaffolding. Two runs selected:
+  </p>
+  <ul style="font-family: var(--mono); font-size: 0.85rem;">
+    <li><strong>A</strong> &middot; {{ stem_a }} &middot;
+        <a href="/api/runs/{{ stem_a }}/frames.json" target="_blank">frames.json</a> &middot;
+        <a href="/figs/{{ stem_a }}.html" target="_blank">original plotly</a></li>
+    <li><strong>B</strong> &middot; {{ stem_b }} &middot;
+        <a href="/api/runs/{{ stem_b }}/frames.json" target="_blank">frames.json</a> &middot;
+        <a href="/figs/{{ stem_b }}.html" target="_blank">original plotly</a></li>
+  </ul>
+  <p style="color: var(--faint); font-size: 0.85rem; font-style: italic;">
+    Three.js side-by-side animation UI will render here (next phase).
+  </p>
+</section>
+
+<script src="/static/theme.js?v=11"></script>
+</body>
+</html>

app/web/templates/index.html

@@ -4,7 +4,7 @@
   <meta charset="utf-8" />
   <meta name="viewport" content="width=device-width,initial-scale=1" />
   <title>embedding notebook</title>
-  <link rel="stylesheet" href="/static/style.css?v=19" />
+  <link rel="stylesheet" href="/static/style.css?v=20" />
   <script src="https://unpkg.com/htmx.org@2.0.4"></script>
   <script type="importmap">
   {
@@ -301,6 +301,13 @@
       </span>
     </div>
 
+    <div class="compare-bar">
+      <button type="button" id="compare-btn" disabled>
+        compare selected <span id="compare-count">(0/2)</span>
+      </button>
+      <span class="compare-hint muted">pick two embeddings &rarr; side-by-side animation in a new tab</span>
+    </div>
+
     <div
       id="runs-slot"
       hx-get="/runs"
@@ -400,6 +407,7 @@
 <script src="/static/theme.js?v=11"></script>
 <script type="module" src="/static/dataset-picker.js?v=11"></script>
 <script type="module" src="/static/metrics.js?v=11"></script>
+<script src="/static/compare-select.js?v=1"></script>
 <script>
   // Anchor-links alone don't expand <details>; force it.
   document.querySelector('a[href="#metrics"]')?.addEventListener('click', () => {