feat(audio): multitrack stem recording — Stage 2 (wire into the mixer)

Wires MultitrackRecorder into the live audio path, behind a recording_mode.

- config: RecordingMode { Mixed, Multitrack, Both } + AppConfig.recording_mode
  (serde-default Mixed, back-compat); CoreCommand::SetRecordingMode, sent at
  app startup from config.
- multitrack.rs: mic now arrives async via push_mic into an internal FIFO,
  drained one frame per end_cycle (mirrors recorder.rs) so the mic track tracks
  the cycle clock; added dir() accessor. mic is a plain WavWriter now.
- core: parallel `multitrack` slot + `is_multitrack` fast-path gate (exactly one
  of the mixed/multitrack recorders is active). SetRecording start branches on
  mode: Mixed → single-file Recorder (unchanged); Multitrack/Both → a per-session
  dir, MultitrackRecorder, and registers everyone already in the room (named,
  silence-aligned from t=0). The mixer taps each peer's RAW frame (pre-volume/
  mute/limiter) into stems and writes peer stems + mix (Both) + end_cycle per
  cycle; the capture thread pushes mic to whichever recorder; PeerJoined adds a
  late joiner's stem track. stop_recording finalizes both.

No UI yet to pick the mode (Stage 3) — defaults to Mixed, so behaviour is
unchanged until then; set recording_mode in config.json to exercise stems.
168 lib tests, clippy --all-targets clean.

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

src/config.rs

@@ -44,6 +44,41 @@ impl RoomLayout {
         [RoomLayout::ThreeColumn, RoomLayout::BottomDock, RoomLayout::Drawer];
 }
 
+/// What a call recording captures. `Mixed` is the original single-file behaviour;
+/// `Multitrack`/`Both` write per-peer stems for post-production (see
+/// `docs/multitrack-recording-plan.md`).
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
+pub enum RecordingMode {
+    /// One mixed WAV (your mic + the incoming mix). Smallest; the default.
+    #[default]
+    Mixed,
+    /// One WAV per peer + your mic, all sample-aligned. Rebuild the mix yourself.
+    Multitrack,
+    /// Per-peer stems + your mic + a convenience mixed track.
+    Both,
+}
+
+impl RecordingMode {
+    /// All variants, in picker display order.
+    pub const ALL: [RecordingMode; 3] =
+        [RecordingMode::Mixed, RecordingMode::Multitrack, RecordingMode::Both];
+
+    /// True when this mode writes per-peer stem tracks (Multitrack or Both).
+    pub fn is_multitrack(self) -> bool {
+        matches!(self, RecordingMode::Multitrack | RecordingMode::Both)
+    }
+}
+
+impl std::fmt::Display for RecordingMode {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(match self {
+            RecordingMode::Mixed => "Mixed (single file)",
+            RecordingMode::Multitrack => "Multitrack (per-peer stems)",
+            RecordingMode::Both => "Both (stems + mixed)",
+        })
+    }
+}
+
 impl std::fmt::Display for RoomLayout {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         f.write_str(match self {
@@ -141,6 +176,9 @@ pub struct AppConfig {
     /// Chosen UI colour theme.
     #[serde(default)]
     pub theme: AppTheme,
+    /// What a call recording captures (mixed / per-peer stems / both).
+    #[serde(default)]
+    pub recording_mode: RecordingMode,
     #[serde(default)]
     pub custom_sound_self_join: Option<String>,
     #[serde(default)]
@@ -195,6 +233,7 @@ impl Default for AppConfig {
             chat_drawer_width: default_chat_drawer_width(),
             room_layout: RoomLayout::default(),
             theme: AppTheme::default(),
+            recording_mode: RecordingMode::default(),
             custom_sound_self_join: None,
             custom_sound_peer_join: None,
             custom_sound_peer_leave: None,