fix(webrtc): SO_SNDBUF 2MB + VBV rate limiting + stats integration

P0 - UDP send buffer: set SO_SNDBUF=2MB to prevent EAGAIN on large IDR
frames (218KB/256KB keyframes caused 18+ EAGAIN bursts). Actual Linux
buffer 4096KB confirmed.

P1 - VBV rate limiting: cap rc_max_rate=bitrate and rc_buffer_size=
bitrate/4 for WebRTC encode path, preventing oversized IDR frames.

Stats: integrate PipelineStats into cap_portal (dropped_count), state.rs
(wlroots path), webrtc.rs (browser getStats enhancement + stats panel).

src/cap_portal.rs

@@ -73,6 +73,7 @@ pub struct CapPortal {
     event_rx: Receiver<PwCtrlEvent>,
     pw_thread: Option<JoinHandle<()>>,
     rt: Runtime,
+    pw_dropped: Arc<AtomicU64>,
 }
 
 /// PipeWire 捕获线程的上下文数据
@@ -149,6 +150,7 @@ impl CapPortal {
             event_rx,
             pw_thread: Some(pw_thread),
             rt,
+            pw_dropped,
         })
     }
 
@@ -160,6 +162,16 @@ impl CapPortal {
         &self.event_rx
     }
 
+    /// Returns the total number of PipeWire frames dropped due to channel backlog.
+    pub fn dropped_count(&self) -> u64 {
+        self.pw_dropped.load(Ordering::Relaxed)
+    }
+
+    /// Returns the number of frames currently waiting in the capture channel.
+    pub fn capture_queue_depth(&self) -> usize {
+        self.frame_rx.len()
+    }
+
     /// 通过 XDG Desktop Portal 建立屏幕录制会话
     ///
     /// 与桌面环境的 D-Bus 服务交互，请求用户授权屏幕录制。
@@ -476,7 +488,9 @@ fn pipewire_thread(ctx: PwThreadCtx) {
     let mainloop = match pw::main_loop::MainLoopBox::new(None) {
         Ok(ml) => ml,
         Err(e) => {
-            let _ = event_tx.try_send(PwCtrlEvent::Error(format!("MainLoop::new failed: {e}")));
+            if let Err(e) = event_tx.try_send(PwCtrlEvent::Error(format!("MainLoop::new failed: {e}"))) {
+                tracing::error!("MainLoop::new failed and error channel also failed: {e}");
+            }
             return;
         }
     };
@@ -484,7 +498,9 @@ fn pipewire_thread(ctx: PwThreadCtx) {
     let context = match pw::context::ContextBox::new(mainloop.loop_(), None) {
         Ok(c) => c,
         Err(e) => {
-            let _ = event_tx.try_send(PwCtrlEvent::Error(format!("Context::new failed: {e}")));
+            if let Err(e) = event_tx.try_send(PwCtrlEvent::Error(format!("Context::new failed: {e}"))) {
+                tracing::error!("Context::new failed and error channel also failed: {e}");
+            }
             return;
         }
     };
@@ -492,7 +508,9 @@ fn pipewire_thread(ctx: PwThreadCtx) {
     let core = match context.connect_fd(pw_fd, None) {
         Ok(c) => c,
         Err(e) => {
-            let _ = event_tx.try_send(PwCtrlEvent::Error(format!("connect_fd failed: {e}")));
+            if let Err(e) = event_tx.try_send(PwCtrlEvent::Error(format!("connect_fd failed: {e}"))) {
+                tracing::error!("connect_fd failed and error channel also failed: {e}");
+            }
             return;
         }
     };
@@ -514,7 +532,9 @@ fn pipewire_thread(ctx: PwThreadCtx) {
     ) {
         Ok(s) => s,
         Err(e) => {
-            let _ = event_tx.try_send(PwCtrlEvent::Error(format!("Stream::new failed: {e}")));
+            if let Err(e) = event_tx.try_send(PwCtrlEvent::Error(format!("Stream::new failed: {e}"))) {
+                tracing::error!("Stream::new failed and error channel also failed: {e}");
+            }
             return;
         }
     };
@@ -584,12 +604,14 @@ fn pipewire_thread(ctx: PwThreadCtx) {
 
                 let raw_buf = unsafe { stream.dequeue_raw_buffer() };
                 if raw_buf.is_null() {
+                    tracing::trace!("process: null raw_buf");
                     return;
                 }
 
                 // 获取 SPA buffer 结构体，包含数据数组、元数据等
                 let spa_buf = unsafe { (*raw_buf).buffer };
                 if spa_buf.is_null() {
+                    tracing::trace!("process: null spa_buf");
                     unsafe { stream.queue_raw_buffer(raw_buf) };
                     return;
                 }
@@ -599,6 +621,7 @@ fn pipewire_thread(ctx: PwThreadCtx) {
                 let n_datas = unsafe { (*spa_buf).n_datas };
                 let datas_ptr = unsafe { (*spa_buf).datas };
                 if n_datas == 0 || datas_ptr.is_null() {
+                    tracing::trace!("process: no data (n_datas={n_datas})");
                     unsafe { stream.queue_raw_buffer(raw_buf) };
                     return;
                 }
@@ -609,11 +632,13 @@ fn pipewire_thread(ctx: PwThreadCtx) {
                     unsafe { &*(datas_ptr as *const pw::spa::buffer::Data) };
                 let fd = data_ref.fd();
                 if fd < 0 {
+                    tracing::trace!("process: invalid fd={fd}");
                     unsafe { stream.queue_raw_buffer(raw_buf) };
                     return;
                 }
 
                 if data_ref.as_raw().chunk.is_null() {
+                    tracing::trace!("process: null chunk");
                     unsafe { stream.queue_raw_buffer(raw_buf) };
                     return;
                 }
@@ -651,6 +676,7 @@ fn pipewire_thread(ctx: PwThreadCtx) {
                     return;
                 };
                 if width == 0 || height == 0 || format == 0 {
+                    tracing::trace!("process: invalid dimensions {width}x{height} format={format}");
                     unsafe { stream.queue_raw_buffer(raw_buf) };
                     return;
                 }
@@ -695,7 +721,9 @@ fn pipewire_thread(ctx: PwThreadCtx) {
         StreamFlags::AUTOCONNECT | StreamFlags::MAP_BUFFERS,
         &mut params,
     ) {
-        let _ = event_tx.try_send(PwCtrlEvent::Error(format!("stream.connect failed: {e}")));
+        if let Err(e) = event_tx.try_send(PwCtrlEvent::Error(format!("stream.connect failed: {e}"))) {
+            tracing::error!("stream.connect failed and error channel also failed: {e}");
+        }
         return;
     }