feat(portal): BWE-driven resolution adaptation + duplicate frame skipping

WebRTC client bandwidth estimate now drives both encoder bitrate and
resolution tier selection, replacing the previous static-target encoder.

- webrtc.rs: enable str0m BWE (seeded at 5 Mbps), surface
  EgressBitrateEstimate + KeyframeRequest events, expose
  get_bwe_estimate() / set_need_keyframe()
- state_portal.rs: wire bitrate/resolution channels between the WebRTC
  thread and the encode thread; tier ladder [1440p, 1080p, 720p] with
  downscale at 60% budget and upscale hysteresis (120% sustained 10s)
- avhw.rs: SwEncImport::poll_resolution_commands() rebuilds the import
  filter graph on UpdateResolution; SwEncEncode::recreate_encoder()
  rebuilds sws/enc_video/yuv_frame atomically; hash_sampled_y_plane()
  skips duplicate frames; VBV x264opts cap IDR bursts; H.264 level 4.0
  (muxer) / 4.2 (WebRTC)
- state.rs: sync wlr-screencopy GOP to fps*2 max 20 for parity
- fix: drain bitrate_rx + resolution_rx BEFORE the stride check in
  encode_cpu_frame() so the new (smaller-stride) frame produced after
  a resolution change does not hit the stale (larger) enc_width and
  crash the encode thread
- WebRTC GOP widened to fps*2 max 20 (was fps/2 max 10)

src/fps_limit.rs

@@ -82,7 +82,12 @@ mod tests {
             }
         }
 
-        assert!(outputs.len() >= 3, "expected at least 3 outputs, got {} ({:?})", outputs.len(), outputs);
+        assert!(
+            outputs.len() >= 3,
+            "expected at least 3 outputs, got {} ({:?})",
+            outputs.len(),
+            outputs
+        );
         assert_eq!(outputs[0], 0);
     }