Function overlay_run 

pub fn overlay_run(
    wt_root: &Path,
    project: &Path,
    tmp: &Path,
    files: &[(HeldOutFile, String)],
    runner: &Runner,
    policy: &Policy,
) -> Result<Vec<Ran>, String>

Writes each held-out file over the worktree, runs its command, and puts the tree back exactly as it was: no file of the set, nothing a test wrote, no changed judge input survives — a later attempt can read none of it.