Record PrimaryRegistry delegator ownership via a MessageService accepted-delivery
hook (won the session lock + queued delivery), never at bridge_send request time, so
a concurrent sender that times out BUSY cannot steal a live turn's reply routing.
Make Rendezvous.open atomic fail-if-present so a double open trips loudly instead of
replacing the waiter another send is blocked on. Answering a bridge_ask keeps the same
ownership (no rewrite). Adds ownership/rendezvous regression tests.
Dai Ha
b39f700765