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.