feat(bridge): alt(331f) 写通道——build 4 + CLI raw --channel alt，测试全绿

tests/test_cli.py

@@ -28,6 +28,9 @@ class FakeTransport:
     async def write_rxtx(self, data, response=True):
         self.rxtx_writes.append(data)
 
+    async def write_alt(self, data, response=True):
+        self.rxtx_writes.append(data)
+
     async def request_device_id(self, timeout=8.0):
         return "DEVICE12345678"