feat: P3-T4 relay presence registry — gossip fingerprints across relay mesh

PresenceRegistry tracks who is connected where:
- register_local/unregister_local for directly connected users
- update_peer for fingerprints reported by peer relays
- lookup returns Local or Remote(addr)
- expire_stale removes entries older than timeout

Gossip via probe connections:
- New SignalMessage::PresenceUpdate { fingerprints, relay_addr }
- Probes send local fingerprints every 10s alongside Ping/Pong
- Receiving relay updates its remote presence table

HTTP API on metrics port:
- GET /presence — all known fingerprints + locations
- GET /presence/:fingerprint — single lookup
- GET /peers — peer relays + their connected users

Wired into relay main:
- Registry created at startup
- register_local after auth+handshake
- unregister_local on disconnect
- Passed to probe mesh and metrics server

Also marks FC-10 as DONE in integration tracker.

48 relay tests + 42 proto tests passing.

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

crates/wzp-proto/src/packet.rs

@@ -591,6 +591,16 @@ pub enum SignalMessage {
     },
     /// Acknowledge a transfer request.
     TransferAck,
+
+    /// Presence update from a peer relay (gossip protocol).
+    /// Sent periodically over probe connections to share which fingerprints
+    /// are connected to the sending relay.
+    PresenceUpdate {
+        /// Fingerprints currently connected to the sending relay.
+        fingerprints: Vec<String>,
+        /// Address of the sending relay (e.g., "192.168.1.10:4433").
+        relay_addr: String,
+    },
 }
 
 /// Reasons for ending a call.
@@ -776,6 +786,40 @@ mod tests {
         assert!(matches!(decoded, SignalMessage::TransferAck));
     }
 
+    #[test]
+    fn presence_update_signal_roundtrip() {
+        let msg = SignalMessage::PresenceUpdate {
+            fingerprints: vec!["aabb".to_string(), "ccdd".to_string()],
+            relay_addr: "10.0.0.1:4433".to_string(),
+        };
+        let json = serde_json::to_string(&msg).unwrap();
+        let decoded: SignalMessage = serde_json::from_str(&json).unwrap();
+        match decoded {
+            SignalMessage::PresenceUpdate { fingerprints, relay_addr } => {
+                assert_eq!(fingerprints.len(), 2);
+                assert!(fingerprints.contains(&"aabb".to_string()));
+                assert!(fingerprints.contains(&"ccdd".to_string()));
+                assert_eq!(relay_addr, "10.0.0.1:4433");
+            }
+            _ => panic!("expected PresenceUpdate variant"),
+        }
+
+        // Empty fingerprints list
+        let msg_empty = SignalMessage::PresenceUpdate {
+            fingerprints: vec![],
+            relay_addr: "10.0.0.2:4433".to_string(),
+        };
+        let json = serde_json::to_string(&msg_empty).unwrap();
+        let decoded: SignalMessage = serde_json::from_str(&json).unwrap();
+        match decoded {
+            SignalMessage::PresenceUpdate { fingerprints, relay_addr } => {
+                assert!(fingerprints.is_empty());
+                assert_eq!(relay_addr, "10.0.0.2:4433");
+            }
+            _ => panic!("expected PresenceUpdate variant"),
+        }
+    }
+
     #[test]
     fn fec_ratio_encode_decode() {
         let ratio = 0.5;