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-proto/src/packet.rs

@@ -665,21 +665,14 @@ pub enum SignalMessage {
         tls_fingerprint: String,
     },
 
-    /// 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 {
+    /// Federation: this relay now has local participants in a global room.
+    GlobalRoomActive {
         room: String,
     },
 
-    /// Federation: local participant list changed for a federated room.
-    FederationParticipantUpdate {
+    /// Federation: this relay's last local participant left a global room.
+    GlobalRoomInactive {
         room: String,
-        participants: Vec<RoomParticipant>,
     },
 }