feat: rewrite desktop audio I/O with lock-free ring buffers

- Replace Mutex-based CPAL callbacks with atomic SPSC ring buffers
- Proper async send/recv loops (no block_on), 20ms playout tick
- Add signal task for RoomUpdate presence display
- Add --alias, --raw-room flags and key persistence (~/.wzp/identity)
- Add SetAlias signal variant and relay-side handling
- Graceful Ctrl+C shutdown with force-quit on second press

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

crates/wzp-proto/src/packet.rs

@@ -653,6 +653,11 @@ pub enum SignalMessage {
         /// List of participants currently in the room.
         participants: Vec<RoomParticipant>,
     },
+
+    /// Set or update the client's display name.
+    /// Sent by client after joining; relay updates the participant entry and
+    /// re-broadcasts a RoomUpdate to all participants.
+    SetAlias { alias: String },
 }
 
 /// A participant entry in a RoomUpdate message.