Function is_stale 

pub fn is_stale(cache: &Path, reference_mtimes: &[PathBuf]) -> bool

Consider a cache “stale” if it is older than the command’s fdl.yml (config changes), or older than a sentinel binary path when supplied.

Missing cache ⇒ stale (return true). Missing reference mtime ⇒ treat the cache as fresh (conservative: don’t refresh what we can’t justify).