feat(audio): multitrack stem recording — Stage 1 (pure core + plan)

Scopes the differentiating "record every peer to their own synced track"
feature and lands its pure, isolated core (no live-audio wiring yet).

- docs/multitrack-recording-plan.md: scope contract + locked decisions
  (raw stems pre-volume/mute, stems + a mixed track, silence-pad late joiners).
- src/audio/multitrack.rs: MultitrackRecorder over the existing WavWriter.
  One master clock = the mixer cycle; every end_cycle() appends exactly
  FRAME_SAMPLES to every track (silence where idle) so all stems stay
  sample-aligned. add_peer back-pads a late joiner to cycle 0; track_filename
  gives fs-safe `<slug>-<shortid>.wav` (reuses sanitize_name). Optional mix
  track for "Both" mode.
- +5 unit tests: equal length across tracks, late-joiner leading silence,
  stems-only omits mix, fit() pad/truncate, filename slugging/disambiguation.

Stage 2 (wire into the mixer) is next, behind a checkpoint. 168 lib tests,
clippy --all-targets clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/audio/mod.rs

@@ -54,6 +54,7 @@ pub trait AudioBackend: Send + Sync {
 pub mod echo_cancel;
 pub mod gate;
 pub mod limiter;
+pub mod multitrack;
 pub mod pipewire_impl;
 pub mod pw_cli;
 pub mod recorder;