Function spawn_parser_thread 

pub fn spawn_parser_thread(
    stderr: ChildStderr,
    alias: String,
    tx: Sender<ParserMessage>,
    stderr_buffer: Arc<Mutex<VecDeque<String>>>,
    stop: Arc<AtomicBool>,
) -> JoinHandle<()>

Spawn the per-tunnel stderr parser thread. Reads the child’s stderr line-by-line, captures channel events into tx and stores the last few raw lines in stderr_buffer for exit-reason display.