feat(relay+client): relay reports build version in Ack

Add relay_build field to RegisterPresenceAck so the client logs
which relay version it connected to. Shows in the debug log as
register_signal:ack_received {"relay_build":"f843a93"}.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-relay/tests/multi_reflect.rs

@@ -65,6 +65,7 @@ async fn spawn_mock_relay() -> (SocketAddr, tokio::task::JoinHandle<()>) {
                                 .send_signal(&SignalMessage::RegisterPresenceAck {
                                     success: true,
                                     error: None,
+                                    relay_build: None,
                                 })
                                 .await;
                         }