feat: federation rewrite — global rooms router model

Major rewrite of relay federation replacing virtual participants with
a clean router model:

1. Global rooms: [[global_rooms]] in TOML config declares rooms that
   are bridged across federation. Each relay is a router + local SFU.

2. Room events: RoomManager emits LocalJoin/LocalLeave via broadcast
   channel when rooms transition between empty and non-empty.

3. GlobalRoomActive/Inactive signals: relays announce when they have
   local participants in global rooms. Peers track active state and
   forward media accordingly. Announcements propagate for multi-hop.

4. Media forwarding: separated from SFU loop. Local participant sends
   via mpsc channel → egress task → forward_to_peers() → room-hash
   tagged datagrams to active peer links. Inbound datagrams delivered
   to local participants + forwarded to other active peers (multi-hop).

5. Loop prevention: don't forward back to source relay.

6. Room name hashing: is_global_room() checks both plain name and
   hash (clients hash room names for SNI privacy).

Removed: ParticipantSender::Federation, federated_participants, virtual
participant join/leave, periodic room polling. Rooms now only contain
local participants.

Signaling tested: 3-relay chain (A→B←C) correctly propagates
GlobalRoomActive through B to both A and C. Media forwarding plumbing
in place but needs final debugging.

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

crates/wzp-relay/src/config.rs

@@ -25,6 +25,13 @@ pub struct TrustedConfig {
     pub label: Option<String>,
 }
 
+/// A room declared global — bridged across all federated peers.
+#[derive(Clone, Debug, Serialize, Deserialize)]
+pub struct GlobalRoomConfig {
+    /// Room name to bridge (e.g., "android").
+    pub name: String,
+}
+
 /// Configuration for the relay daemon.
 ///
 /// All fields have defaults, so a minimal TOML file only needs the
@@ -73,6 +80,9 @@ pub struct RelayConfig {
     /// Federation peer relays.
     #[serde(default)]
     pub peers: Vec<PeerConfig>,
+    /// Global rooms bridged across federation.
+    #[serde(default)]
+    pub global_rooms: Vec<GlobalRoomConfig>,
     /// Trusted relay fingerprints — accept inbound federation from these relays.
     /// Unlike [[peers]], no url is needed — the peer connects to us.
     #[serde(default)]
@@ -99,6 +109,7 @@ impl Default for RelayConfig {
             ws_port: None,
             static_dir: None,
             peers: Vec::new(),
+            global_rooms: Vec::new(),
             trusted: Vec::new(),
             debug_tap: None,
         }