feat(sim): two-element audio crossfade keyed by scrub fraction

Per plan Task 2. Adds <audio id=aud-b>; blendAudio(lo,frac) mixes the two
adjacent scale soundtracks by knob fraction (element reuse across crossings so
only the genuinely-new scale reloads); restAudio settles a single scale at rest.
Diagnostic readout now reports whichever element is audible.

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

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

@@ -29,6 +29,12 @@ async function wheelOnStage(page: Page, deltaY: number) {
   await page.locator("#stage").dispatchEvent("wheel", { deltaY });
 }
 
+test("two audio elements exist for crossfade", async ({ page }) => {
+  await boot(page);
+  const n = await page.evaluate(() => document.querySelectorAll("audio#aud, audio#aud-b").length);
+  expect(n).toBe(2);
+});
+
 test("zoom in plays the matching morph, lands locked, and stays put while parked", async ({ page }) => {
   await boot(page);
   const start = (await page.locator("#scale-name").textContent())!;