Function sweep_runtime_base 

pub fn sweep_runtime_base(runtime_base: &Path, max_age: Duration)

Remove runtime layers left under runtime_base by earlier runs that exited without their RuntimeLayer drop (a crash or a hard kill). Only entries older than max_age are removed, so a concurrent run’s fresh layer is never touched. Junctions are unlinked rather than followed, so linked game data is never deleted. Best-effort: inaccessible entries are left alone.