Function start_recording 

pub fn start_recording(
    app: AppHandle,
    recording_state: State<'_, RecordingState>,
    preview_state: State<'_, PreviewState>,
    camera_handle: State<'_, CameraPipelineHandle>,
    mic_state: State<'_, MicCaptureState>,
    mic_handle: State<'_, MicCaptureHandle>,
    config: RecordingConfig,
) -> Result<u64, String>

Start a coordinated recording session (M-RECORD.1 of M-RECORD-EXPORT).

Spawns each enabled per-channel pipeline (camera / screen / microphone / system audio) inside one RecordingSession orchestrator. Picker selections (camera_id, microphone_id, screen_source_id) are threaded through to the existing per- channel start paths so M-CAM.4 / M-MIC.3 / M-SCK.0.1 routing applies inside a session too.

Rollback discipline: if any one stream fails to start, the session aborts — the streams that already started are stopped best-effort and the function returns Err.

Lifecycle: session enters Starting. The per-channel pipelines transition Starting → Running independently as each produces its first frame; the recording-status event push (M-RECORD.1 follow-up commit) will roll those up into the master Running transition.

Errors

• "a recording session is already active" — re-entrant call without a prior stop_recording. Idempotent-by-design rather than implicit-replace because mid-session changes invalidate the M-EXPORT encoder state.
• "no streams enabled — pick at least one input" — caller passed SessionStreams { camera: false, ... }.
• "screen + system audio capture require macOS 13.0+" — non-macOS caller enabled either of those channels.
• Underlying per-channel start error, prefixed with the channel name (e.g. "camera start failed: ...").