feat(portal): independent WebRTC thread + channel tuning for 60fps mouse latency

- Move WebRTC send to dedicated wl-webrtc-webrtc thread (was inline in main loop)
- Reduce frame_rx 16→1, input_tx 2→1 (drop-on-full), webrtc_tx 32→2
- Recv_timeout 10ms→2ms to reduce pipeline latency
- Fix sent_gap_p95 stats bug: compute gap at actual send time in WebRTC
  thread instead of batch-draining at snapshot time (was always 0.0ms)
- High profile via AVCodecContext.profile, veryfast preset, 5x bitrate
- Stats drain via sent_gap channel with record_send_from_thread()
- Shutdown: drop input_tx → join encode → drop webrtc_tx → join webrtc

src/cap_portal.rs

@@ -98,7 +98,7 @@ impl CapPortal {
     /// 执行流程:
     ///   1. 创建 Tokio 运行时（用于异步 Portal 调用）
     ///   2. 通过 XDG Desktop Portal 请求屏幕录制权限，获取 PipeWire fd 和 node_id
-    ///   3. 创建有界通道（容量 16）用于帧传递
+    ///   3. 创建有界通道（容量 1）用于帧传递（最新帧优先，避免队列积压延迟）
     ///   4. 创建 eventfd 对，用于线程安全的关闭信号传递
     ///   5. 启动 PipeWire 捕获线程
     pub fn new(args: &Args) -> Result<Self> {
@@ -107,7 +107,7 @@ impl CapPortal {
         let no_persist = args.no_persist;
         let (pw_fd, node_id) = rt.block_on(async { Self::setup_portal(no_persist).await })?;
 
-        let (frame_tx, frame_rx) = bounded(16);
+        let (frame_tx, frame_rx) = bounded(1);
         let (event_tx, event_rx) = bounded(8);
 
         let efd = unsafe { libc::eventfd(0, libc::EFD_CLOEXEC | libc::EFD_NONBLOCK) };
@@ -722,11 +722,12 @@ fn pipewire_thread(ctx: PwThreadCtx) {
                     pts,
                 };
 
-                if let Err(crossbeam_channel::TrySendError::Full(_)) = frame_tx.try_send(frame) {
-                    let prev = dropped.fetch_add(1, Ordering::Relaxed);
-                    if prev > 0 && prev % 30 == 0 {
-                        tracing::warn!("dropped {prev} frames total: encoder backlog");
+                match frame_tx.try_send(frame) {
+                    Ok(()) => {}
+                    Err(crossbeam_channel::TrySendError::Full(_)) => {
+                        dropped.fetch_add(1, Ordering::Relaxed);
                     }
+                    Err(crossbeam_channel::TrySendError::Disconnected(_)) => {}
                 }
                 unsafe { stream.queue_raw_buffer(raw_buf) };
             }