fix(state): add WebRTC support to wlr-screencopy backend

Fixes #1 -- --port mode with wlr-screencopy backend caused panic at
negotiate_format() because self.args.output is None and .expect() was
called unconditionally.

Changes:
- Introduce StreamingEncoder enum wrapping EncState (MP4) and
  SwEncState (WebRTC) with unified frames_rgb/encode_frame/flush API
- Add WebRTC fields to State<S> (webrtc, webrtc_tx, webrtc_rx,
  webrtc_frames_sent) matching Portal backend pattern
- State::new() returns Result<Self> for clean WebRtcState init failure
- negotiate_format() branches on webrtc_tx: WebRTC path uses
  SwEncState::new_webrtc(), MP4 path unchanged (hardware VAAPI)
- Add poll_webrtc() method to drive signaling + channel drain
- Event loop calls poll_webrtc() each iteration
- Fix pre-existing test/bench Args construction (Option<String> output,
  missing no_persist field)

src/main.rs

@@ -100,7 +100,7 @@ fn run_wlr_screencopy(args: Args) -> Result<()> {
     let qhandle = queue.handle();
     // State 是 wlr-screencopy 后端的核心状态机，
     // 内部管理输出探测、截屏请求、编码器构建、帧采集等阶段
-    let mut state = State::new(gm, args, qhandle);
+    let mut state = State::new(gm, args, qhandle)?;
 
     // Extract the Wayland fd and consume any immediately-available events.
     // prepare_read() flushes outgoing requests; read() pulls whatever the
@@ -246,6 +246,8 @@ fn run_wlr_screencopy(args: Args) -> Result<()> {
         // - Streaming: 正常采集中，请求下一帧
         state.queue_alloc_frame();
 
+        state.poll_webrtc()?;
+
         // 状态机遇到致命错误时退出
         if state.errored {
             tracing::error!("Fatal error in state machine, exiting");