Function prune 

pub fn prune(
    paths: &DataPaths,
    plan: &Plan,
    in_use: &dyn Fn() -> InUse,
) -> Report

Remove what plan_prune found, re-checking each directory at the last moment: in_use is asked again (a CLI may have started since the plan), and a shared install’s lock must be free. The completion marker goes FIRST, so a directory half removed by a crash is already “not an install” and the next install into it clears the rest.