fix(audio): off/on Audio + Video toggles; fix Safari autoplay + GPU video-off

Operator-driven live-UI revision:
- Video + Audio are now Dev-Mode-style on/off toggles (Audio on = soundtrack);
  white-noise deferred (removed from the live control; pure machinery kept).
- FIX video-off not blanking: the <video>/<canvas> are GPU-composited and could
  paint above the auto-z #black on a real GPU — give #black z-index + its own
  layer AND hide the video layers (opacity 0) on video-off.
- FIX no soundtrack on Safari/iOS: Safari unlocks <audio> only when play() runs
  INSIDE the user gesture; the start gesture now primes both A/B elements
  synchronously, so the async crossfades are allowed to play.
- Toggles wired on 'change' (matches the Dev Mode switch).
- player/audio.py AUDIO_SOURCES -> {off, soundtrack}; tests + E2E updated.
- Verified in headless Chromium AND WebKit: audio plays, video-off blanks both
  layers, zero JS errors. 288 tests + 3 Playwright E2E pass.

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

simulator/app.py

@@ -39,9 +39,6 @@ STATIC_DIR = Path(__file__).parent / "static"
 MEDIA_DIR = Path(__file__).parent / "sample_media"
 DEFAULT_MANIFEST = MEDIA_DIR / "manifest.json"
 
-# The global white-noise bed (audio spec §5.2): synthesized, altitude-independent.
-NOISE_URL = "/media/audio/noise/pink.mp3"
-
 # Per-process boot token: changes on every server restart so the dev live-reload
 # (below) also fires when Python code changes (which needs a restart), not just
 # when a static asset's mtime changes.
@@ -190,7 +187,7 @@ def create_app(manifest_path: Optional[Path] = None) -> FastAPI:
         scale_audio = ""
         if app.state.ring is not None:
             scale_audio = scale_at(app.state.ring, req.altitude_index).audio
-        audio = resolve_audio(c.audio, scale_audio=scale_audio, noise_url=NOISE_URL)
+        audio = resolve_audio(c.audio, scale_audio=scale_audio)
         return {
             "plan": render_plan_to_dict(plan),
             "render": {