feat: relay federation infrastructure — room bridging, loop prevention, peer connections

Phase 1 of relay federation:

1. Signal messages: FederationRoomJoin/Leave/ParticipantUpdate added
   to SignalMessage enum for relay-to-relay room coordination.

2. Room changes: ParticipantOrigin (Local/Federated) tracking, loop
   prevention (federated media only forwards to local participants),
   ParticipantSender::Federation with 8-byte room-hash prefixed
   datagrams, merged participant lists (local + remote), new methods:
   join_federated(), update_federated_participants(), local_senders(),
   active_rooms(), local_participants().

3. FederationManager: connects to configured peers via QUIC with SNI
   "_federation", reconnects with exponential backoff (5s-300s),
   exchanges FederationRoomJoin signals, runs recv loops for both
   signals and media datagrams, creates virtual participants in rooms.

4. Accept-side: _federation SNI handling in main.rs, unknown peer
   gets helpful "add to relay.toml" log message, recognized peers
   handed off to FederationManager.

TODO: TLS fingerprint verification — currently outbound connections
use client_config() which doesn't present a cert, so inbound
verification fails. Need mutual TLS or URL-based peer matching.

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

crates/wzp-proto/src/packet.rs

@@ -656,6 +656,25 @@ pub enum SignalMessage {
         /// List of participants currently in the room.
         participants: Vec<RoomParticipant>,
     },
+
+    // ── Federation signals (relay-to-relay) ──
+
+    /// Federation: a room exists on the sending relay with active local participants.
+    FederationRoomJoin {
+        room: String,
+        participants: Vec<RoomParticipant>,
+    },
+
+    /// Federation: a room is now empty on the sending relay.
+    FederationRoomLeave {
+        room: String,
+    },
+
+    /// Federation: local participant list changed for a federated room.
+    FederationParticipantUpdate {
+        room: String,
+        participants: Vec<RoomParticipant>,
+    },
 }
 
 /// A participant entry in a RoomUpdate message.