Function writer_thread 

fn writer_thread(
    rx: Receiver<Tap>,
    file: File,
    path: String,
    backend: &'static str,
    mode_name: &'static str,
    audio: Option<AudioTrackConfig>,
    shared: Arc<RecShared>,
)

Writer-thread body: drain the queue, convert Annex-B to AVCC, and mux. Output starts at the first IDR with parameter sets; earlier frames are discarded and audio packets held. Runs until every sender is dropped (stop) or a write error occurs, then finalizes the file and publishes the final status.