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/cap_portal.rs

@@ -54,6 +54,8 @@ pub struct PwDmaBufFrame {
 pub enum PwCtrlEvent {
     /// 流已结束（PipeWire 流断开连接或进入错误状态）
     StreamEnded,
+    /// Format/dimensions changed mid-stream
+    FormatChanged { width: u32, height: u32 },
     /// 发生错误，包含错误描述信息
     Error(String),
 }
@@ -554,7 +556,13 @@ fn pipewire_thread(ctx: PwThreadCtx) {
                 pw::stream::StreamState::Unconnected => {
                     let _ = event_tx_state.try_send(PwCtrlEvent::StreamEnded);
                 }
-                _ => {}
+                pw::stream::StreamState::Paused => {
+                    tracing::warn!("PipeWire stream paused (compositor may be switching content)");
+                }
+                pw::stream::StreamState::Streaming => {
+                    tracing::info!("PipeWire stream (re)started");
+                }
+                pw::stream::StreamState::Connecting => {}
             }
         })
         // 参数变化回调（格式协商）
@@ -562,6 +570,7 @@ fn pipewire_thread(ctx: PwThreadCtx) {
         // id 为参数类型，param 包含具体的格式参数（分辨率、像素格式等）
         .param_changed({
             let format_info = format_info.clone();
+            let event_tx = event_tx.clone();
             move |_, _, id, param| {
                 // 仅处理 Format 类型的参数变化
                 let Some(param) = param else { return };
@@ -583,7 +592,18 @@ fn pipewire_thread(ctx: PwThreadCtx) {
                 let framerate = info.framerate();
                 let max_framerate = info.max_framerate();
                 // 保存协商后的格式信息，供 process 回调读取
+                let previous_format = format_info.get();
                 format_info.set(Some((width, height, drm_format, modifier)));
+                if let Some((previous_width, previous_height, _, _)) = previous_format {
+                    if width != previous_width || height != previous_height {
+                        tracing::warn!(
+                            "PipeWire dimensions changed: {}x{} (format renegotiation)",
+                            width,
+                            height
+                        );
+                        let _ = event_tx.try_send(PwCtrlEvent::FormatChanged { width, height });
+                    }
+                }
                 tracing::info!(
                     "PipeWire format negotiated: {width}x{height}, \
                      drm_format={drm_format:#010x}, modifier={modifier:#x}, \