[真实链路自测]：完成完整流程自测和ACK缓冲修正，包含真实Provider验收、播放回声清理和回归测试

src/owner_voice_pet/transport.py

@@ -58,12 +58,15 @@ class MemoryAudioTransport:
         frames: list[AudioFrame] | None = None,
         input_available: bool = True,
         output_available: bool = True,
+        flush_clears_input: bool = True,
     ) -> None:
         self._frames: deque[AudioFrame] = deque(frames or [])
         self.played_segments: list[AudioSegment] = []
+        self.flush_count = 0
         self.started = False
         self._input_available = input_available
         self._output_available = output_available
+        self._flush_clears_input = flush_clears_input
 
     def start_input(
         self, device_id: str | None = None, sample_rate: int = 16000, channels: int = 1
@@ -104,6 +107,9 @@ class MemoryAudioTransport:
         return PlaybackResult(True, segment.duration_ms)
 
     def flush_input(self) -> int:
+        self.flush_count += 1
+        if not self._flush_clears_input:
+            return 0
         count = len(self._frames)
         self._frames.clear()
         return count