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-client/src/featherchat.rs

@@ -111,9 +111,8 @@ pub fn signal_to_call_type(signal: &SignalMessage) -> CallSignalType {
         SignalMessage::SessionForwardAck { .. } => CallSignalType::Offer, // reuse
         SignalMessage::RoomUpdate { .. } => CallSignalType::Offer, // reuse
         SignalMessage::FederationHello { .. }
-        | SignalMessage::FederationRoomJoin { .. }
-        | SignalMessage::FederationRoomLeave { .. }
-        | SignalMessage::FederationParticipantUpdate { .. } => CallSignalType::Offer, // relay-only
+        | SignalMessage::GlobalRoomActive { .. }
+        | SignalMessage::GlobalRoomInactive { .. } => CallSignalType::Offer, // relay-only
     }
 }