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-relay/src/main.rs

@@ -392,7 +392,6 @@ async fn main() -> anyhow::Result<()> {
             room_mgr.clone(),
             endpoint.clone(),
             tls_fp.clone(),
-            metrics.clone(),
         ));
         let fm_run = fm.clone();
         tokio::spawn(async move { fm_run.run().await });