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/lib.rs

@@ -4,8 +4,8 @@ pub mod backend_detect;
 pub mod cap_portal;
 pub mod cap_wlr_screencopy;
 pub mod fps_limit;
-pub mod stats;
 pub mod state;
 pub mod state_portal;
+pub mod stats;
 pub mod transform;
 pub mod webrtc;