Function register_ack 

fn register_ack(
    packet_id: Result<PacketId, ClientError>,
    queued_tx: Option<Sender<Result<(), Error>>>,
    completion_tx: Sender<Result<Vec<ReasonCode>, Error>>,
    client: &Client<JournaledSession>,
    pending: &mut Pending,
    queued_waiting: &mut Vec<Sender<Result<(), Error>>>,
)

Tracks a recorded SUBSCRIBE/UNSUBSCRIBE’s completion future. The caller pumps the session afterwards, which puts the group on the wire and arms its ack deadline via TransmitEvent::SubscribeSent / TransmitEvent::UnsubscribeSent. Shared tail of send_subscribe and send_unsubscribe, which differ only in how packet_id is obtained.