debug(voice): add VAD start/stop events to trace where audio pipeline breaks

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

voice.py

@@ -355,7 +355,15 @@ class VoiceSession:
                 vad=_get_vad(),
             )
 
-            # Debug: log speech events
+            # Debug: log speech pipeline events to pinpoint where audio is lost
+            @self.session.on("user_started_speaking")
+            def _on_speaking_start():
+                logger.info("VAD: user_started_speaking")
+
+            @self.session.on("user_stopped_speaking")
+            def _on_speaking_stop():
+                logger.info("VAD: user_stopped_speaking")
+
             @self.session.on("user_speech_committed")
             def _on_user_speech(msg):
                 logger.info("USER_SPEECH: %s", msg.text_content)