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>

docs/INTEGRATION_TASKS.md

@@ -77,7 +77,9 @@ Based on featherChat commit 65f6390 — FUTURE_TASKS.md with WZP integration ite
 ### WZP-FC-7. Missed call notifications — TODO (0.5d)
 ### WZP-FC-8. Cross-project identity verification — DONE (15 tests, 26dc848)
 ### WZP-FC-9. HKDF salt investigation — DONE (no mismatch)
-### WZP-FC-10. Web bridge shared auth — TODO (1-2d)
+### WZP-FC-10. Web bridge shared auth — DONE
+- FC: GET /v1/wzp/relay-config, CORS layer, service token
+- WZP: web bridge --auth-url validates browser tokens via FC
 ### FC-CRATE-1. Standalone warzone-protocol — DONE (v0.0.21, 4a4fa9f)
 
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