feat: federated presence — RoomUpdate includes remote participants

GlobalRoomActive signal now carries participant list from the
announcing relay. When received, the relay:
1. Stores remote participants per peer link
2. Broadcasts merged RoomUpdate to local clients (local + all remote)

This means clients on different relays can now SEE each other in the
participant list. Also fixes build: removed non-existent metric field
references that were added by linter.

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

crates/wzp-proto/src/packet.rs

@@ -668,6 +668,9 @@ pub enum SignalMessage {
     /// Federation: this relay now has local participants in a global room.
     GlobalRoomActive {
         room: String,
+        /// Participants on the announcing relay (for federated presence).
+        #[serde(default)]
+        participants: Vec<RoomParticipant>,
     },
 
     /// Federation: this relay's last local participant left a global room.