feat(a11y): reduced-motion freeze-to-stills with OS-default toggle

Single guard in playLoop() holds a still frame; autoScrub jumps instantly;
applyReduceMotion pauses/resumes. Default from prefers-reduced-motion, then
the choice persists. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

simulator/e2e/tests/a11y.spec.ts

@@ -40,3 +40,23 @@ test.describe("Task 4 — Photosensitivity warning gate", () => {
     await expect(page.locator("#run-sim")).toBeVisible();
   });
 });
+
+test.describe("Task 5 — Reduced motion", () => {
+  test("reduce-motion toggle pauses video playback", async ({ page }) => {
+    await page.emulateMedia({ reducedMotion: "no-preference" }); // deterministic default-off
+    await page.goto("/");
+    await page.locator("#motion-warning-continue").click().catch(() => {});
+    const rm = page.locator("#reduce-motion");
+    await expect(rm).not.toBeChecked();          // default-off under no-preference
+    await page.locator("#run-sim").click();
+    await page.waitForTimeout(500);
+    await expect
+      .poll(() => page.locator("#vid-loop").evaluate((v: HTMLVideoElement) => v.paused))
+      .toBe(false);                              // experience running → video plays
+    await page.locator('label[for="reduce-motion"]').click(); // hidden input: toggle via its label
+    await expect(rm).toBeChecked();
+    await expect
+      .poll(() => page.locator("#vid-loop").evaluate((v: HTMLVideoElement) => v.paused))
+      .toBe(true);                               // reduced motion → frozen
+  });
+});

simulator/static/app.js

@@ -258,6 +258,7 @@ function loadLoop(clip) {
 }
 
 function playLoop() {
+  if (isReduced()) return;     // reduced motion holds a still frame — never auto-play the loop
   if (loopVid.paused) loopVid.play().catch(() => {});
 }
 
@@ -677,6 +678,11 @@ const PER_ALTITUDE_MS = 1200;
 let autoRaf = 0;
 function autoScrub(targetPos, perAltMs = PER_ALTITUDE_MS) {
   if (!ring || ring.scales.length < 2) return;
+  if (isReduced()) {            // reduced motion: no autonomous tween — jump to target + lock
+    if (autoRaf) { cancelAnimationFrame(autoRaf); autoRaf = 0; }
+    setPos(targetPos);
+    return;
+  }
   if (autoRaf) cancelAnimationFrame(autoRaf);
   const fromPos = pos, dist = targetPos - fromPos;
   if (!dist) { setPos(targetPos); return; }
@@ -847,7 +853,8 @@ function setPos(next) {
     busy = false;
     delete vid.dataset.morph;  // clear so re-entering the same segment reloads the morph (not the base)
     currentClipId = null;      // force ensureClipMedia to reload + loop the locked clip
-    playLoop();                // the loop element was preloaded to this clip @ tail during the scrub → instant, no reload
+    if (!isReduced()) playLoop();   // reduced motion holds the landing frame; else run the base loop
+    else { vid.pause(); loopVid.pause(); }
     showActiveSource();
     setNeedle(ringIndex * dialStep());
     renderScaleReadout();
@@ -917,9 +924,42 @@ function jumpToScale(idx) {
 // cache) — no server endpoints. Editing labels stays in /author.html.
 const DEV_KEY = "hef.devMode";
 const WARN_KEY = "hef.motionWarnDismissed";
+const RM_KEY = "hef.reduceMotion";
 let devMode = false;
 let devStatsTimer = null;
 
+// --- Reduced motion (freeze-to-stills) ---------------------------------------
+// Default follows the OS `prefers-reduced-motion` until the visitor chooses, then
+// their choice persists. When ON: video holds a frame (paused), auto transitions
+// jump instantly instead of tweening — knob changes still re-grade the still.
+let reduceMotion = false;
+function isReduced() { return reduceMotion; }
+function initReduceMotion() {
+  const box = $("reduce-motion");
+  let stored = null;
+  try { stored = localStorage.getItem(RM_KEY); } catch (_) {}
+  const prefers = window.matchMedia && window.matchMedia("(prefers-reduced-motion: reduce)").matches;
+  reduceMotion = stored === null ? !!prefers : stored === "1";
+  if (box) {
+    box.checked = reduceMotion;
+    box.addEventListener("change", () => {
+      reduceMotion = box.checked;
+      try { localStorage.setItem(RM_KEY, reduceMotion ? "1" : "0"); } catch (_) {}
+      applyReduceMotion();
+    });
+  }
+  applyReduceMotion();
+}
+function applyReduceMotion() {
+  if (reduceMotion) {
+    if (autoRaf) { cancelAnimationFrame(autoRaf); autoRaf = 0; }
+    vid.pause();
+    loopVid.pause();
+  } else if (videoEverOn && $("visual").checked) {
+    playLoop();                         // resume the held loop on opt-out
+  }
+}
+
 // One-time photosensitivity notice shown over the stage before the experience
 // begins. Independent of media load; gated visually by the loading splash, then
 // dismissed-once (persisted) so returning visitors go straight to "Run simulation".
@@ -1284,6 +1324,7 @@ async function main() {
   buildDial();                // draw the altitude knob from the ring's scales
   initDev();                  // wire the Dev Mode toggle + pool picker (reads persisted state)
   initLanguage();             // populate the language dropdown + wire live switching
+  initReduceMotion();         // reduced-motion state + toggle (default from OS pref)
   renderScaleReadout();
   // Sliders stream on "input"; the toggles fire on "change".
   for (const id of ["left", "right", "mood"]) $(id).addEventListener("input", debounced);