M11.2: audio playback + waveforms + audio effects

- Audio effects (Backwards, Bass & Treble, Delay, Flange & Chorus,
  High-Low Pass, Modulator, Parametric EQ, Reverb, Stereo Mixer, Tone)
  applied per layer in the comp mixdown, so export and preview share them.
  RBJ-cookbook biquads, Schroeder/Moorer reverb, modulated delay lines;
  stateless per block via pre-roll. Tone makes any layer audible.
- Real-time audio preview: ui-egui AudioFeed/AudioPlayback (feeder thread
  mixing ahead) behind an AudioDevice hook; the desktop app implements it
  with cpal (L6). Audio clock drives playback (consumed frames minus device
  latency, looped through the work area). Preview panel Include Audio /
  playback.muteAudio.
- Audio panel VU meters with peak hold and clip indicators; selected
  layer's Audio Levels.
- Timeline Audio > Waveform for footage with audio (L reveals Audio
  Levels, LL the Waveform), drawn from a cached per-item peak summary.
- ui.playback reports audio state and meter levels.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/control-protocol.md

@@ -43,7 +43,7 @@ Useful query commands for `engine.execute` (they aren't journaled or undoable):
 | `ui.elements` | `{prefix?}` | `[{id, label, rect: [x, y, w, h]}]`: the widgets drawn in the last frame, in points. |
 | `ui.set` | `{tool?, workspace?, theme?, focused?, viewer?, timeline?, menuBar?, home?}` | `viewer: {zoom (null = fit), res: Full\|Half\|Third\|Quarter\|Auto, pan: [x,y], grid, rulers, safeMargins, transparencyGrid}`; `timeline: {pps (null = fit), start, graphEditor, showModes, openLayers: [ids], openGroups: [uids]}`. |
 | `ui.panel.show` / `ui.panel.close` | `{panel}` | Panels: `Project`, `Composition`, `Timeline`, `EffectControls`, `EffectsPresets`, `Info`, `Preview`, `Character`, `Paragraph`, `Align` and others. |
-| `ui.playback` | `{action: play\|stop\|toggle}` | `{playing, time}` |
+| `ui.playback` | `{action: play\|stop\|toggle\|status}` | `{playing, time, audio, levelsDb, peaksDb}` (audio: an audio preview is running; L/R meter levels and held peaks in dBFS) |
 | `ui.resize` | `{width, height}` | Resizes the window (points). |
 | `ui.focus` | `{}` | Activates the window and takes keyboard focus. Agents normally avoid this, because the app is started without stealing focus. |
 | `app.quit` | `{}` | Closes the app. |