tick

One retransmission tick: the analogue of utp_socket_impl::tick(now). If the oldest unacked packet has gone unacknowledged for longer than packetTimeoutMicros, an RTO has fired. We then retransmit every in-flight packet (go-back-N) and deepen the backoff. After MAX_TIMEOUTS fruitless rounds we fail the socket, so a dead peer cannot block a reader forever. Built under lock; the resends go out after.