Function stop_recording 

pub fn stop_recording(
    app: AppHandle,
    recording_state: State<'_, RecordingState>,
    mic_state: State<'_, MicCaptureState>,
    mic_handle: State<'_, MicCaptureHandle>,
) -> Result<RecordingSummary, String>

Stop the active recording session (M-RECORD.1).

Tears down each enabled per-channel pipeline in reverse start order. The returned RecordingSummary carries the final per-stream tally + the encoded file path (M-EXPORT.4 populates the path; today it’s None).

Errors

• "no recording session is active" — caller invoked stop without a matching start.