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
+  });
+});