feat: multi-party rooms (SFU) + push-to-talk radio mode

Room-based SFU relay:
- Clients join named rooms (room name from QUIC SNI)
- Each participant's packets forwarded to all others (no mixing)
- Multiple rooms run concurrently on one relay
- Web bridge passes room name from URL path to relay

Push-to-talk (radio mode):
- Toggle "Radio mode" checkbox after connecting
- Hold PTT button or spacebar to transmit
- Release to mute mic (receive-only)
- Works on desktop (spacebar) and mobile (touch)

URL routing:
- /myroom → joins room "myroom"
- Room name input field as fallback

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

crates/wzp-relay/src/lib.rs

@@ -10,6 +10,7 @@
 pub mod config;
 pub mod handshake;
 pub mod pipeline;
+pub mod room;
 pub mod session_mgr;
 
 pub use config::RelayConfig;