Sync watcher.stop() leaks the gRPC stream thread

[zeevdr]

Description

sdk/src/opendecree/watcher.py:184-191 joins the thread with a 5-second timeout but never calls stream.cancel(). The gRPC Subscribe call blocks forever inside the thread; with daemon=True the thread leaks until process exit.

Acceptance criteria

• Store self._stream and call self._stream.cancel() before thread.join
• Add a regression test that asserts no thread leak after watcher.stop()