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>

Cargo.lock

@@ -3507,7 +3507,7 @@ dependencies = [
 
 [[package]]
 name = "warzone-protocol"
-version = "0.0.21"
+version = "0.0.38"
 dependencies = [
  "base64",
  "bincode",