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>

Cargo.toml

@@ -67,6 +67,7 @@ egui = "0.36"
 eframe = { version = "0.36", default-features = false }
 egui_kittest = "0.36"
 rfd = "0.17"
+cpal = "0.16"
 
 # Local co-development against a FilmCraft checkout next to this repo: uncomment.
 # [patch."https://github.com/storytold/filmcraft"]