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/avhw.rs

@@ -25,6 +25,9 @@ use crate::transform::{transpose_if_transform_transposed, Transform};
 pub enum BitrateCommand {
     UpdateBitrate { target_bps: u64 },
     UpdateResolution { width: u32, height: u32 },
+    /// Force the next encoded frame to be an IDR. Sent by the WebRTC thread
+    /// in response to str0m `Event::KeyframeRequest` or a resolution change.
+    ForceKeyframe,
 }
 
 #[derive(Clone, Copy, Debug)]
@@ -856,6 +859,7 @@ impl SwEncImport {
                     requested = Some((width & !1, height & !1));
                 }
                 BitrateCommand::UpdateBitrate { .. } => {}
+                BitrateCommand::ForceKeyframe => {}
             }
         }
 
@@ -966,6 +970,10 @@ pub struct SwEncEncode {
     fps: u32,
     bitrate: u64,
     gop_size: u32,
+    /// Set true when WebRTC requests a keyframe. Forces the next frame to
+    /// `AV_PICTURE_TYPE_I` and bypasses the dedup hash check. Cleared only
+    /// after `avcodec_send_frame` accepts the forced frame.
+    force_keyframe_pending: bool,
 }
 
 const FNV1A_OFFSET_BASIS: u64 = 0xcbf29ce484222325;
@@ -1028,6 +1036,7 @@ impl SwEncEncode {
             fps,
             bitrate,
             gop_size,
+            force_keyframe_pending: false,
         })
     }
 
@@ -1066,6 +1075,7 @@ impl SwEncEncode {
             fps,
             bitrate,
             gop_size,
+            force_keyframe_pending: false,
         })
     }
 
@@ -1103,9 +1113,15 @@ impl SwEncEncode {
                     }
                 }
                 BitrateCommand::UpdateResolution { .. } => {}
+                BitrateCommand::ForceKeyframe => {
+                    self.force_keyframe_pending = true;
+                    tracing::debug!("encode thread: ForceKeyframe requested");
+                }
             }
         }
 
+        let force_this_frame = self.force_keyframe_pending;
+
         while let Ok(change) = self.resolution_rx.try_recv() {
             self.recreate_encoder(change.width, change.height)?;
         }
@@ -1130,7 +1146,7 @@ impl SwEncEncode {
         self.frame_count = self.frame_count.saturating_add(1);
         let current_hash = hash_sampled_y_plane(&frame.y_data, width, height, frame.y_stride);
         let force_gop_frame = self.gop_size > 0 && frame_index % u64::from(self.gop_size) == 0;
-        if frame_index > 0 && !force_gop_frame && current_hash == self.last_frame_hash {
+        if frame_index > 0 && !force_gop_frame && !force_this_frame && current_hash == self.last_frame_hash {
             tracing::debug!(frame_index, "skipping duplicate frame");
             self.last_frame_hash = current_hash;
             return Ok(());
@@ -1172,8 +1188,16 @@ impl SwEncEncode {
         let start_ts = self.starting_timestamp.unwrap_or(0);
 
         // SAFETY: yuv_frame is initialized, writable, and matches the opened encoder format.
+        // pict_type is reset every frame: the AVFrame is reused, so without resetting to NONE
+        // a previously-forced I-type would leak into subsequent P-frames. With forced-idr=1
+        // set on the encoder, AV_PICTURE_TYPE_I produces a true IDR NALU.
         unsafe {
             (*self.yuv_frame).pts = pts;
+            (*self.yuv_frame).pict_type = if force_this_frame {
+                ffi::AVPictureType::AV_PICTURE_TYPE_I
+            } else {
+                ffi::AVPictureType::AV_PICTURE_TYPE_NONE
+            };
             let ret = ffi::avcodec_send_frame(self.enc_video.as_mut_ptr(), self.yuv_frame);
             if ret < 0 {
                 bail!(
@@ -1183,6 +1207,10 @@ impl SwEncEncode {
             }
         }
 
+        if force_this_frame {
+            self.force_keyframe_pending = false;
+        }
+
         self.drain_encoder(start_ts)
     }
 
@@ -1215,6 +1243,7 @@ impl SwEncEncode {
         self.enc_height = height;
         self.last_frame_hash = 0;
         self.frame_count = 0;
+        self.force_keyframe_pending = true;
 
         Ok(())
     }
@@ -1741,6 +1770,14 @@ fn create_software_h264_encoder(
             // avcodec_alloc_context3. Setting level is a simple i32 field
             // assignment on a properly aligned struct.
             (*enc.as_mut_ptr()).level = 42; // H.264 Level 4.2 (up to 1440p@30)
+            // SAFETY: priv_data belongs to the unopened libx264 encoder context.
+            // `forced-idr` is an FFmpeg-level private option (not x264-native),
+            // so it must be set via av_opt_set, NOT via the x264opts string.
+            // With forced-idr=1, setting AV_PICTURE_TYPE_I on an input frame
+            // produces a true IDR NALU with inline SPS/PPS (repeat_headers=1).
+            let key = CString::new("forced-idr").unwrap();
+            let val = CString::new("1").unwrap();
+            ffi::av_opt_set((*enc.as_mut_ptr()).priv_data, key.as_ptr(), val.as_ptr(), 0);
             let key = CString::new("x264opts").unwrap();
             let vbv_maxrate = bitrate;
             let vbv_bufsize = bitrate / 4;