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/webrtc.rs

@@ -200,6 +200,7 @@ struct WebRtcInner {
     video_pt: Option<Pt>,
     connected: bool,
     need_keyframe: bool,
+    force_keyframe_to_encode: bool,
     current_bwe_estimate: Option<Bitrate>,
     rtp_clock: u32,
     buf: Vec<u8>,
@@ -353,6 +354,17 @@ impl WebRtcState {
     pub fn set_need_keyframe(&mut self) {
         if let Some(inner) = self.inner.as_mut() {
             inner.need_keyframe = true;
+            inner.force_keyframe_to_encode = true;
+        }
+    }
+
+    pub fn take_force_keyframe(&mut self) -> bool {
+        if let Some(inner) = self.inner.as_mut() {
+            let v = inner.force_keyframe_to_encode;
+            inner.force_keyframe_to_encode = false;
+            v
+        } else {
+            false
         }
     }
 }
@@ -429,6 +441,7 @@ impl WebRtcInner {
             video_pt: None,
             connected: false,
             need_keyframe: false,
+            force_keyframe_to_encode: false,
             current_bwe_estimate: None,
             rtp_clock: 0,
             buf: vec![0u8; 65535],
@@ -446,6 +459,7 @@ impl WebRtcInner {
             .map_err(|e| anyhow::anyhow!("accept_offer: {e}"))?;
 
         self.need_keyframe = true;
+        self.force_keyframe_to_encode = true;
         tracing::info!("SDP exchange complete, waiting for ICE/DTLS...");
 
         self.discover_video_params();
@@ -503,6 +517,7 @@ impl WebRtcInner {
                             tracing::info!("WebRTC connected!");
                             self.connected = true;
                             self.need_keyframe = true;
+                            self.force_keyframe_to_encode = true;
                             self.discover_video_params();
                         }
                         Event::IceConnectionStateChange(IceConnectionState::Disconnected) => {
@@ -525,6 +540,7 @@ impl WebRtcInner {
                         Event::KeyframeRequest(_) => {
                             tracing::info!("received keyframe request from viewer");
                             self.need_keyframe = true;
+                            self.force_keyframe_to_encode = true;
                         }
                         Event::EgressBitrateEstimate(est) => {
                             let bitrate = match est {