stripTrailingSep preserves root

Trap: unconditionally stripping or appending a separator looks like straightforward path normalization, but it can turn / into an empty path or create //, breaking directory-boundary process.execPath.startsWith() checks. Fix: stripTrailingSep() strips only when p.length > 1; known binary paths must end in exactly one separator. samePath() tolerates one trailing separator and folds case only on Windows and macOS.