fix(state_portal): prevent shutdown deadlock on full bounded channel (closes #8)

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

@@ -935,13 +935,22 @@ 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()) {
-                            tracing::warn!(
-                                "WebRTC channel send failed (receiver dropped): {} bytes lost",
-                                e.0.len()
-                            );
-                            self.webrtc_disconnected = true;
-                            break;
+                        match tx.try_send(data.to_vec()) {
+                            Ok(()) => {}
+                            Err(crossbeam_channel::TrySendError::Full(frame)) => {
+                                tracing::warn!(
+                                    "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;
+                            }
                         }
                     }
                 }