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>

player/controls.py

@@ -2,7 +2,7 @@
 
 This is the serial-contract payload shared with sub-project 4 (firmware). It
 models the full panel from the audio spec §3: a Visual toggle (on/off) + an
-Audio source selector (off/soundtrack/white_noise), the four experience knobs
+Audio source selector (off/soundtrack; white-noise/music deferred), the four experience knobs
 (0..4), and the two intensity levels (volume, brightness). The wire framing
 itself is the separate 3<->4 serial contract; this module is the *decoded* form.
 """