Make OFF run-directory initialization OS-agnostic on Windows via pathlib

[Copilot]

OFF aborted on Windows when OFF_PATH was unset because directory fallback logic depended on os.environ["PWD"] (Unix-specific). This change switches fallback resolution to pathlib + Path.cwd() so run-directory initialization works consistently across platforms.

• Root path resolution in OFF.__dir_init__

  • Replaced string-based path construction with Path composition.
  • Kept precedence: explicit data folder → OFF_PATH/runs → cwd-based fallback.
  • Preserved existing 03_Code special case using Path.cwd().name == "03_Code" and cwd.parent / "runs".

• Directory and root tracking cleanup

  • Replaced brittle string slicing ([:-len("runs/")]) with root_path.parent.
  • Normalized sim_dir construction via root_path / f"off_run_{run_id}".
  • Switched directory creation to Path(...).mkdir(parents=True, exist_ok=True).

• Focused regression coverage

  • Added tests that validate fallback behavior when OFF_PATH is missing:
    • cwd fallback to <cwd>/runs
    • 03_Code fallback to <parent>/runs
  • Tests intentionally set PWD to an unrelated value to verify logic no longer depends on it.

cwd = Path.cwd()
if cwd.name == "03_Code":
    root_path = Path(data_dir) if data_dir else cwd.parent / "runs"
else:
    root_path = Path(data_dir) if data_dir else cwd / "runs"

[ivovs-tud]

Please remove