Function validate_threads_root_path_safety 

pub fn validate_threads_root_path_safety(
    path: &Path,
    heddle_dir: &Path,
    threads_root: &Path,
    reserved_regions: &[(PathBuf, Option<PathBuf>)],
) -> Result<(), ThreadsRootPathSafetyError>

Pure layout + reserved-region validation for a worktree target under the threads root / heddle storage policy.

reserved_regions is (region_root, exempt_exact_path) pairs: candidate is nested when it starts with region_root unless it equals exempt_exact (self-thread re-materialize exemption). Callers enumerate durable thread records; this stays FS-free.