multiport: carry a lost install race's counter to the surviving lane window

Two routines can race on a lane's first packet and derive a session each. Only
one gets installed, and the loser was dropped along with the replay-window entry
for the packet it had just accepted, leaving that one counter replayable.

The two sessions hold identical keys, so the winner's window is the same window
in every respect that matters: mark the counter seen on it instead. Closes the
gap without putting a lock on the decrypt path.

connection_state.go

@@ -168,6 +168,15 @@ func (cs *ConnectionState) Decrypt(l *slog.Logger, messageCounter uint64, packet
 	return out, nil
 }
 
+// noteSeen records a counter that some other session for the same keys already
+// accepted, so a packet doesn't become replayable just because the session that
+// decrypted it was thrown away. See laneSet.installSession, its only caller.
+func (cs *ConnectionState) noteSeen(l *slog.Logger, messageCounter uint64) {
+	cs.decryptLock.Lock()
+	cs.window.Update(l, messageCounter)
+	cs.decryptLock.Unlock()
+}
+
 func (cs *ConnectionState) VerifyRelay(l *slog.Logger, messageCounter uint64, packet []byte, nb []byte) error {
 	cs.decryptLock.Lock()
 	result := cs.window.Check(l, messageCounter)