fix(sonar): document empty _Sink.flush (python:S1186)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

core/voice_bundle.py

@@ -65,7 +65,8 @@ class _Sink:
     def __init__(self): self.buf = bytearray(); self.pos = 0
     def write(self, b): self.buf += b; self.pos += len(b); return len(b)
     def tell(self): return self.pos
-    def flush(self): pass
+    def flush(self):
+        pass  # in-memory buffer, nothing to flush; zipfile just calls it
     def drain(self):
         out = bytes(self.buf); self.buf.clear(); return out