GameInvariants checks card counts, unique instance ids and damage. The CLI stress command runs random legal bot games and fails on the first violation. It exposed a real bug: an optional OnGuard ability that could not be paid (or was declined) reset the step from Guard to None while the battle was still open. OfferAbility now reports Skipped/Paused/Resolved and callers continue only when needed. 1000/1000 games finish with no errors.
