Function spawn_accept_unix 

pub fn spawn_accept_unix(
    listener: UnixListener,
    handler: Arc<dyn Handler>,
    subs: SubRegistry,
    conn_counter: Arc<AtomicU64>,
    write_timeout: Duration,
) -> Result<()>

Spawn the background accept thread for listener: one blocking thread per connection, each running handle_conn against handler. Peers arrive in the PeerOrigin::Management trust domain (they dialed a listener). Returns once the accept thread is spawned; a thread-spawn failure is surfaced as the error.