feat(sim): audio on/off toggle -> 0-10 level dial; first video lifts it to 3/10

Operator request. Replaces the Audio checkbox with a 0-10 range slider that sets the
master soundtrack gain (audioVol = level/10), scaling both the per-scale rest volume
and the two-track scrub crossfade. Level 0 = silent. Turning Video on for the first
time now raises audio to 3/10 (was: full on) so the experience opens gently; raising
the slider from 0 unlocks playback in-gesture (Safari). E2E: dial is range 0-10
default 0, and first video-on lifts it to 3/10.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

simulator/e2e/tests/altitude-lock.spec.ts

@@ -29,13 +29,13 @@ async function wheelOnStage(page: Page, deltaY: number) {
   await page.locator("#stage").dispatchEvent("wheel", { deltaY });
 }
 
-// The Audio toggle is a visually-hidden checkbox styled as a switch; set it + fire
-// `change` directly (headless Chromium relaxes the autoplay gesture requirement).
+// The Audio control is a 0–10 level slider; set it to full + fire `input` directly
+// (headless Chromium relaxes the autoplay gesture requirement).
 async function enableAudio(page: Page) {
   await page.evaluate(() => {
     const c = document.querySelector("#audio") as HTMLInputElement;
-    c.checked = true;
-    c.dispatchEvent(new Event("change", { bubbles: true }));
+    c.value = "10";
+    c.dispatchEvent(new Event("input", { bubbles: true }));
   });
 }
 
@@ -55,6 +55,27 @@ test("two audio elements exist for crossfade", async ({ page }) => {
   expect(n).toBe(2);
 });
 
+test("audio is a 0-10 level dial, defaulting to 0", async ({ page }) => {
+  await boot(page);
+  const ctl = await page.evaluate(() => {
+    const c = document.querySelector("#audio") as HTMLInputElement;
+    return { type: c.type, min: c.min, max: c.max, value: c.value };
+  });
+  expect(ctl.type).toBe("range");
+  expect(ctl.min).toBe("0");
+  expect(ctl.max).toBe("10");
+  expect(ctl.value).toBe("0");                 // silent until raised
+});
+
+test("turning video on for the first time lifts audio to 3/10", async ({ page }) => {
+  await boot(page);
+  expect(await page.evaluate(() => (document.querySelector("#audio") as HTMLInputElement).value)).toBe("0");
+  await enableVideo(page);                       // first video-on couples audio at a gentle level
+  await expect.poll(() => page.evaluate(() => (document.querySelector("#audio") as HTMLInputElement).value)).toBe("3");
+  // The level label reflects it.
+  expect(await page.evaluate(() => document.querySelector("#audio-level-val")?.textContent)).toBe("3");
+});
+
 test("dragging the dial scrubs morph currentTime and audio gains", async ({ page }) => {
   await boot(page);
   await enableAudio(page);                                 // hidden custom toggle → set + dispatch change