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-proto/src/packet.rs

@@ -716,6 +716,9 @@ pub enum SignalMessage {
         success: bool,
         #[serde(skip_serializing_if = "Option::is_none")]
         error: Option<String>,
+        /// Relay's build version (git short hash).
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        relay_build: Option<String>,
     },
 
     /// Direct call offer routed through the relay to a specific peer.

crates/wzp-relay/src/main.rs

@@ -1006,6 +1006,7 @@ async fn main() -> anyhow::Result<()> {
                 let _ = transport.send_signal(&SignalMessage::RegisterPresenceAck {
                     success: true,
                     error: None,
+                    relay_build: Some(BUILD_GIT_HASH.to_string()),
                 }).await;
 
                 info!(%addr, fingerprint = %client_fp, alias = ?client_alias, "signal client registered");

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;
                         }