A02: Atomic cooldown state with exclusive lock and flush

- _write_state: write to temp file, fsync, rename atomically
- Acquire exclusive lock before any file operations
- Flush and fsync before unlock to prevent data loss
- Remove stale .tmp file after successful write
- Add test for atomic write behavior
- 109 Python tests pass

tests/test_community_cli.py

@@ -57,3 +57,20 @@ def test_validate_url_host_rejects_missing_hostname() -> None:
         _validate_url_host("not-a-valid-url")
     with pytest.raises(ValueError, match="valid hostname"):
         _validate_url_host("")
+
+
+def test_write_state_is_atomic_with_flush(tmp_path: Path) -> None:
+    """A02: _write_state uses exclusive lock, flush, and atomic rename."""
+    from rf4_research.community_cli import _read_state, _write_state
+
+    state_file = tmp_path / "state.json"
+    _write_state(state_file, {"key1": "value1"})
+    assert state_file.exists()
+    assert not (state_file.with_suffix(".tmp")).exists()
+    state = _read_state(state_file)
+    assert state == {"key1": "value1"}
+
+    _write_state(state_file, {"key1": "value2", "key2": "value3"})
+    state = _read_state(state_file)
+    assert state == {"key1": "value2", "key2": "value3"}
+    assert not (state_file.with_suffix(".tmp")).exists()