Make sure CityGML is found

src/simstadt/runner.py

@@ -255,6 +255,8 @@ def _resolve_project(citygml_path: Path, project_path: Path | None) -> tuple[Pat
     - CityGML inside a .proj folder: use parent as project, reference by name.
     - Neither: create a temporary repository under /tmp (not auto-cleaned).
     """
+    if not citygml_path.exists():
+        raise FileNotFoundError(f"{citygml_path} not found")
     if project_path is not None:
         project_path.mkdir(exist_ok=True, parents=True)
         dest = project_path / citygml_path.name