fix: stabilize Polygon config loading and sync

tests/test_converter.py

@@ -180,3 +180,45 @@ class TestFileRoundtrip:
             assert len(loaded["cfsParams"]) == 2
         finally:
             os.unlink(fname)
+
+
+class TestGenerationSync:
+    """The CLI output directory should match its source after a clean run."""
+
+    def test_main_removes_yaml_missing_from_source(self, tmp_path, monkeypatch, dummy_yaml):
+        source_dir = tmp_path / "source"
+        output_dir = tmp_path / "output"
+        source_dir.mkdir()
+        output_dir.mkdir()
+        with open(source_dir / "1_dummy.yaml", "w", encoding="utf-8") as source_file:
+            yaml.safe_dump(dummy_yaml, source_file, allow_unicode=True)
+        (output_dir / "27_obsolete.yaml").write_text("old: true\n", encoding="utf-8")
+        (output_dir / "keep.txt").write_text("not YAML\n", encoding="utf-8")
+
+        monkeypatch.setattr(
+            sys,
+            "argv",
+            ["from_stands.py", str(source_dir), str(output_dir)],
+        )
+        from_stands.main()
+
+        assert (output_dir / "1_dummy.yaml").is_file()
+        assert not (output_dir / "27_obsolete.yaml").exists()
+        assert (output_dir / "keep.txt").is_file()
+
+    def test_main_keeps_existing_yaml_when_conversion_fails(self, tmp_path, monkeypatch):
+        source_dir = tmp_path / "source"
+        output_dir = tmp_path / "output"
+        source_dir.mkdir()
+        output_dir.mkdir()
+        (source_dir / "broken.yaml").write_text("broken: [", encoding="utf-8")
+        (output_dir / "27_obsolete.yaml").write_text("old: true\n", encoding="utf-8")
+
+        monkeypatch.setattr(
+            sys,
+            "argv",
+            ["from_stands.py", str(source_dir), str(output_dir)],
+        )
+        from_stands.main()
+
+        assert (output_dir / "27_obsolete.yaml").is_file()