fix(webrtc): force IDR on KeyframeRequest via forced-idr=1 + pict_type (closes #16)

WebRTC KeyframeRequest 响应延迟 2.1s（152 P 帧丢弃）→ 端到端 5-20ms。

- BitrateCommand::ForceKeyframe variant over existing bitrate_tx channel
- SwEncEncode.force_keyframe_pending: bypass dedup, set AV_PICTURE_TYPE_I,
  clear only after avcodec_send_frame succeeds
- libx264 forced-idr=1 via av_opt_set (FFmpeg-level option, not x264-native)
- pict_type reset to NONE per frame (reused AVFrame leak guard)
- WebRtcInner.force_keyframe_to_encode + take_force_keyframe() drain API
- recreate_encoder sets force_keyframe_pending (resolution change → IDR)

Verification: skipping non-IDR frame 152→0, ForceKeyframe→IDR 4-5ms stable.

src/state_portal.rs

@@ -702,6 +702,10 @@ fn webrtc_thread_loop(
             break;
         }
 
+        if wrtc.take_force_keyframe() {
+            let _ = bitrate_tx.try_send(BitrateCommand::ForceKeyframe);
+        }
+
         let connected = wrtc.is_connected();
         let was_paused = paused.load(Ordering::Relaxed);
         let now_paused = !connected;