feat(a11y): keyboard + ARIA slider semantics for the Altitude dial

role=slider + tabindex on the dial; Arrow/Home/End stepping; labels become
Enter/Space buttons; aria-valuenow/valuetext track the committed altitude via
renderDial. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

simulator/e2e/tests/a11y.spec.ts

@@ -60,3 +60,21 @@ test.describe("Task 5 — Reduced motion", () => {
       .toBe(true);                               // reduced motion → frozen
   });
 });
+
+test.describe("Task 6 — Keyboard + ARIA dial", () => {
+  test("altitude dial is a keyboard-operable slider", async ({ page }) => {
+    await page.goto("/");
+    const dial = page.locator("#dial");
+    await expect(dial).toHaveAttribute("role", "slider");
+    await expect(dial).toHaveAttribute("tabindex", "0");
+    await dial.focus();
+    const before = await page.locator("#scale-name").textContent();
+    await dial.press("ArrowDown");               // descend one altitude
+    await expect
+      .poll(async () => page.locator("#scale-name").textContent())
+      .not.toBe(before);
+    await dial.press("Home");                    // jump to the top scale (cosmos)
+    await expect(page.locator("#scale-name")).toHaveText(/cosmos|宇宙/i);
+    await expect(dial).toHaveAttribute("aria-valuenow", "0");
+  });
+});

simulator/static/app.js

@@ -753,6 +753,8 @@ function buildDial() {
     const t = svg("text", {
       x: lx, y: ly, "text-anchor": "middle", "dominant-baseline": "central",
       class: "dial-label", "data-index": String(i),
+      role: "button", tabindex: "0",
+      "aria-label": HEFi18n.pickUiString("scale." + ring.scales[i].id, activeLang),
     }, dial);
     t.textContent = ring.scales[i].id;
   }
@@ -778,6 +780,16 @@ function renderDial() {
   for (const el of dial.querySelectorAll(".dial-label")) {
     el.classList.toggle("active", +el.getAttribute("data-index") === ringIndex);
   }
+  setDialAria();
+}
+
+// Reflect the committed altitude into the dial's slider semantics for assistive tech.
+function setDialAria() {
+  if (!dial || !ring) return;
+  dial.setAttribute("aria-valuemax", String(ring.scales.length - 1));
+  dial.setAttribute("aria-valuenow", String(ringIndex));
+  const s = ring.scales[ringIndex];
+  if (s) dial.setAttribute("aria-valuetext", HEFi18n.pickUiString("scale." + s.id, activeLang));
 }
 
 function dialAngle(e) {
@@ -918,6 +930,27 @@ function jumpToScale(idx) {
   if (d) autoScrub(Math.round(pos) + d);
 }
 
+// Keyboard operation of the dial (role="slider"): arrows step one altitude, Home/End
+// jump to the ends. Enter/Space on a focused label jumps to that scale.
+function onDialKey(e) {
+  if (!ring || ring.scales.length < 2) return;
+  const t = e.target;
+  if (t && t.classList && t.classList.contains("dial-label") && (e.key === "Enter" || e.key === " ")) {
+    e.preventDefault();
+    jumpToScale(+t.getAttribute("data-index"));
+    return;
+  }
+  let handled = true;
+  switch (e.key) {
+    case "ArrowDown": case "ArrowRight": autoScrub(Math.round(pos) + 1); break;  // descend
+    case "ArrowUp":   case "ArrowLeft":  autoScrub(Math.round(pos) - 1); break;  // ascend
+    case "Home": jumpToScale(0); break;
+    case "End":  jumpToScale(ring.scales.length - 1); break;
+    default: handled = false;
+  }
+  if (handled) e.preventDefault();
+}
+
 // --- Dev Mode: pool picker + live analysis, all under one toggle ---
 // Off by default, state persisted in localStorage. Everything here is read from
 // data the renderer already has (clips, ring, the alteration response, the preload
@@ -1351,6 +1384,7 @@ async function main() {
   window.addEventListener("pointermove", onDialMove);
   window.addEventListener("pointerup", onDialUp);
   dial.addEventListener("wheel", onWheel, { passive: false });
+  dial.addEventListener("keydown", onDialKey);   // arrows/Home/End + Enter/Space on labels
   $("stage").addEventListener("wheel", onWheel, { passive: false });
   update();                   // render the initial state (both toggles off → black, silent)
   await preloadAllMedia();    // download all media, updating the loading bar

simulator/static/index.html

@@ -65,7 +65,9 @@
       <fieldset>
         <legend data-i18n="altitude.legend">Altitude</legend>
         <div class="dial-wrap">
-          <svg id="dial" viewBox="0 0 100 100" aria-label="Altitude knob (turn to change scale)"></svg>
+          <svg id="dial" viewBox="0 0 100 100" role="slider" tabindex="0"
+               aria-label="Altitude — turn or use arrow keys to change scale"
+               aria-valuemin="0" aria-valuenow="0" aria-valuetext="cosmos"></svg>
         </div>
         <span id="scale-name" class="scale-name">—</span>
         <p class="hint" data-i18n="altitude.hint">Turn the knob (drag it, or scroll) to change altitude — endless: past the deepest it wraps back up to the highest. Click a label to jump there.</p>