feat: WebSocket support in relay — browsers connect directly, no bridge

Implements WS_RELAY_SPEC.md: relay handles both QUIC and WebSocket clients
in shared rooms, eliminating the wzp-web bridge server.

Room abstraction (room.rs):
- New ParticipantSender enum: Quic(transport) | WebSocket(mpsc::Sender)
- send_raw() sends PCM bytes to either transport type
- join_ws() convenience method for WS clients
- Forwarding loops handle mixed QUIC+WS rooms:
  QUIC→QUIC: send_media (trunked if enabled)
  QUIC→WS: send_raw payload bytes
  WS→QUIC: send_raw wraps in MediaPacket
  WS→WS: send_raw binary

WebSocket handler (ws.rs):
- GET /ws/{room} → WebSocket upgrade via axum
- Auth: first msg {"type":"auth","token":"..."} → validates against FC
- mpsc channel bridges room fan-out to WS binary frames
- Session + presence lifecycle matches QUIC path
- Optional static file serving via --static-dir (tower-http ServeDir)

Config: --ws-port 8080, --static-dir ./static
Proto: MediaHeader::default_pcm() for WS→QUIC wrapping

63 relay + 54 proto tests passing.

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

crates/wzp-relay/src/config.rs

@@ -39,6 +39,11 @@ pub struct RelayConfig {
     /// reducing per-packet QUIC datagram overhead.
     #[serde(default)]
     pub trunking_enabled: bool,
+    /// Port for the WebSocket listener (browser clients connect here).
+    /// If None, WebSocket support is disabled.
+    pub ws_port: Option<u16>,
+    /// Directory to serve static files from (HTML/JS/WASM for web clients).
+    pub static_dir: Option<String>,
 }
 
 impl Default for RelayConfig {
@@ -55,6 +60,8 @@ impl Default for RelayConfig {
             probe_targets: Vec::new(),
             probe_mesh: false,
             trunking_enabled: false,
+            ws_port: None,
+            static_dir: None,
         }
     }
 }