fix(state_portal): replace expect() with bail-style error propagation (closes #9)

shutdown() calls enc.flush() → drain_encoder() → tx.send() on a
crossbeam bounded(32) channel.  If the channel is full and the
receiver (webrtc_rx) is alive but not being drained, send() blocks
forever — a self-deadlock since both ends belong to the same struct.

Two-layer fix:
- avhw.rs: replace tx.send() with tx.try_send(); handle Full (drop
  frame) and Disconnected (set flag) separately.
- state_portal.rs: drop webrtc_rx before flushing in shutdown() so
  try_send returns Disconnected immediately.

Regression tests added for the channel semantics.

src/avhw.rs

@@ -935,16 +935,25 @@ impl SwEncState {
                         let data: &[u8] = unsafe {
                             std::slice::from_raw_parts(raw.data, raw.size as usize)
                         };
-                        if let Err(e) = tx.send(data.to_vec()) {
+                        match tx.try_send(data.to_vec()) {
+                            Ok(()) => {}
+                            Err(crossbeam_channel::TrySendError::Full(frame)) => {
                                 tracing::warn!(
-                                "WebRTC channel send failed (receiver dropped): {} bytes lost",
-                                e.0.len()
+                                    "WebRTC channel full, dropping frame: {} bytes lost",
+                                    frame.len()
+                                );
+                            }
+                            Err(crossbeam_channel::TrySendError::Disconnected(frame)) => {
+                                tracing::warn!(
+                                    "WebRTC channel disconnected: {} bytes lost",
+                                    frame.len()
                                 );
                                 self.webrtc_disconnected = true;
                                 break;
                             }
                         }
                     }
+                }
                 None => {}
             }
         }

src/state_portal.rs

@@ -164,7 +164,8 @@ impl StatePortal {
 
                 // 根据是否启用 WebRTC 选择不同的编码器构造方式
                 let enc = if let Some(ref tx) = self.webrtc_tx {
-                    let paused = self.webrtc_paused.as_ref().expect("webrtc_paused must exist when webrtc_tx exists");
+                    let paused = self.webrtc_paused.as_ref()
+                        .ok_or_else(|| anyhow::anyhow!("internal invariant broken: webrtc_paused missing while WebRTC mode is active"))?;
                     avhw::SwEncState::new_webrtc(
                         &drm_path,
                         frame.width,
@@ -179,9 +180,11 @@ impl StatePortal {
                     )?
                 } else {
                     // MP4 模式：编码输出写入文件
+                    let output_path = self.args.output.as_deref()
+                        .ok_or_else(|| anyhow::anyhow!("--output is required in MP4 file output mode; use --port > 0 for WebRTC mode"))?;
                     avhw::SwEncState::new(
                         &drm_path,
-                        std::path::Path::new(self.args.output.as_deref().expect("output required for MP4 mode")),
+                        std::path::Path::new(output_path),
                         frame.width,
                         frame.height,
                         enc_width,
@@ -320,6 +323,9 @@ impl StatePortal {
     ///
     /// 使用 `enc.take()` 确保编码器只被 flush 一次，即使多次调用也安全（幂等）。
     pub fn shutdown(&mut self) {
+        // 先 drop receiver，使 flush() 中的 try_send() 立即返回 Disconnected
+        // 而非在满通道上阻塞（修复 issue #8 死锁）
+        self.webrtc_rx = None;
         if let Some(mut enc) = self.enc.take() {
             if let Err(e) = enc.flush() {
                 tracing::error!("Flush error during shutdown: {e}");
@@ -553,4 +559,46 @@ mod tests {
         assert_eq!(desc.layers[0].planes[0].pitch, 3840 * 4);
     }
 
+    // ── issue #8 regression ──
+
+    #[test]
+    fn try_send_full_channel_returns_full_not_block() {
+        let (tx, rx) = crossbeam_channel::bounded::<Vec<u8>>(2);
+        tx.send(vec![1]).unwrap();
+        tx.send(vec![2]).unwrap();
+
+        assert!(matches!(
+            tx.try_send(vec![3]),
+            Err(crossbeam_channel::TrySendError::Full(_))
+        ));
+        assert_eq!(rx.len(), 2);
+    }
+
+    #[test]
+    fn try_send_after_rx_dropped_returns_disconnected() {
+        let (tx, rx) = crossbeam_channel::bounded::<Vec<u8>>(2);
+        drop(rx);
+
+        assert!(matches!(
+            tx.try_send(vec![1]),
+            Err(crossbeam_channel::TrySendError::Disconnected(_))
+        ));
+    }
+
+    // given: full bounded channel
+    // when:  rx is dropped, then try_send
+    // expect: Disconnected, not blocking
+    #[test]
+    fn shutdown_rx_drop_prevents_deadlock_on_full_channel() {
+        let (tx, rx) = crossbeam_channel::bounded::<Vec<u8>>(2);
+        tx.send(vec![1]).unwrap();
+        tx.send(vec![2]).unwrap();
+        drop(rx);
+
+        assert!(matches!(
+            tx.try_send(vec![3]),
+            Err(crossbeam_channel::TrySendError::Disconnected(_))
+        ));
+    }
+
 }