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/state_portal.rs

@@ -424,7 +424,7 @@ mod tests {
     #[test]
     fn resolve_drm_device_explicit() {
         let args = Args {
-            output: "test.mp4".to_string(),
+            output: Some("test.mp4".to_string()),
             output_name: None,
             fps: 30,
             codec: "h264".to_string(),
@@ -435,6 +435,7 @@ mod tests {
             verbose: false,
             backend: None,
             port: 0,
+            no_persist: false,
         };
         let result = resolve_drm_device(&args).unwrap();
         assert_eq!(
@@ -446,7 +447,7 @@ mod tests {
     #[test]
     fn resolve_drm_device_none_when_not_specified() {
         let args = Args {
-            output: "test.mp4".to_string(),
+            output: Some("test.mp4".to_string()),
             output_name: None,
             fps: 30,
             codec: "h264".to_string(),
@@ -457,6 +458,7 @@ mod tests {
             verbose: false,
             backend: None,
             port: 0,
+            no_persist: false,
         };
         let result = resolve_drm_device(&args).unwrap();
         assert_eq!(result, None);