Function join_key 

pub fn join_key(dir: &str, key: &str) -> String

Join a manifest-relative key (e.g. a part path, MANIFEST_FILENAME) onto a destination sub-directory. An empty dir returns key unchanged — the common case, since production callers pass "" (the manifest lives at the prefix root). Shared by the destination-verification and resume-reconciliation paths so both speak the same key namespace.