Function classify_path_vs_threads_root 

pub fn classify_path_vs_threads_root(
    path: &Path,
    heddle_dir: &Path,
    threads_root: &Path,
) -> ThreadsRootPathClass

Classify path relative to heddle_dir / threads_root.

Inputs must be absolute normalized paths (no ..). Nested-in-existing- thread checks need reserved regions from the caller; see path_is_nested_in_reserved_region.