fix(portal): compositor stall detection + filler frames + PipeWire state logging

P0: Detect compositor frame delivery stalls (>100ms no frames) and log
    stall/resume events with duration. Rate-limited to 1 warn/sec.

P1: Insert duplicate raw CpuNv12Frame filler during stalls at target fps.
    Keeps WebRTC stream smooth (sent_fps 20-40 instead of 3-5 during
    compositor pauses). Stops after 2s max stale. WebRTC mode only.

P2: Replace silent _ => {} in PipeWire state_changed callback with
    explicit Paused/Streaming/Connecting log messages.

P4: Add PwCtrlEvent::FormatChanged for mid-stream dimension changes.
    param_changed detects resolution renegotiation (skips first call).
    Logs warning in poll_and_encode; full encoder reinit deferred.

Verified: cargo check 0 errors, 70/70 tests, release build, --stats live.

src/bin/sw_encode_bench.rs

@@ -66,6 +66,7 @@ fn receive_first_frame(cap: &CapPortal) -> Result<wl_webrtc::cap_portal::PwDmaBu
         if let Ok(ctrl) = cap.event_receiver().try_recv() {
             match ctrl {
                 PwCtrlEvent::StreamEnded => bail!("PipeWire stream ended before first frame"),
+                PwCtrlEvent::FormatChanged { .. } => {}
                 PwCtrlEvent::Error(e) => bail!("PipeWire error: {e}"),
             }
         }
@@ -328,6 +329,7 @@ fn main() -> Result<()> {
                     eprintln!("PipeWire error after {} frames: {}", frames_encoded, e);
                     break;
                 }
+                PwCtrlEvent::FormatChanged { .. } => {}
             }
         }