Function overlay_red 

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

overlay_run for a red check: each distinct command also runs once on the tree without any held-out file, so a file is red only by the failing tests it adds (speccoder_core::verify::classify_red_added).