Function run_pipeline 

fn run_pipeline(
    app: &AppHandle,
    mic_id: &str,
    native_id: &str,
    cancel: &AtomicBool,
    mixer: Option<&SharedAudioMixer>,
)

The actual worker loop. & borrows let the public-facing spawn move cloned values onto the thread without keeping Self alive on the thread.

mixer is None for preview (meter only) and Some for recording — see MicCapturePipeline::spawn.