cases1

1. 2026-09-27 An empty backup suffix makes `sed -i` portable

   now fails loudly a gate refuses any `sed -i` whose argument is not a `.bak` suffix, so both the GNU-only no-suffix form and the macOS-only empty-suffix form fail the suite before they reach CI

   BehavioralEvolutionExample-Based TestsPre-Promotion Invariant Gates