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/stats.rs

@@ -166,6 +166,17 @@ impl PipelineStats {
         }
     }
 
+    /// Record a frame sent from a background WebRTC thread.
+    /// `gap_ms` is the pre-computed time since the previous send (0.0 = first frame).
+    /// Unlike `record_send`, this does not sample `Instant::now()`, so it remains
+    /// accurate even when batch-drained at stats snapshot time.
+    pub fn record_send_from_thread(&mut self, gap_ms: f64) {
+        if gap_ms > 0.0 {
+            self.sent_gaps_ms.push(gap_ms);
+        }
+        self.sent_frames += 1;
+    }
+
     /// Update PipeWire dropped counter (absolute value from AtomicU64).
     pub fn set_pipewire_dropped(&mut self, total_dropped: u64, prev_dropped: u64) {
         self.pipewire_dropped = total_dropped.saturating_sub(prev_dropped);