Function hash_path_u64 

pub fn hash_path_u64(repo_secret: &RepoSecret, path: &str) -> u64

Computes a 64-bit hash key for a path (file or directory).

Uses SHA-256 over repo_secret || path and interprets the first 8 bytes as a big-endian integer. Big-endian keeps bytewise prefix ordering consistent with numeric ordering when ranges are compared lexicographically.