Reference Fixtures + Compatibility Gate

redknightlois

Parent Epic: #9 ## Why A contract is reliable only when correctness is executable and enforced continuously. ## What we want to achieve - Versioned reference artifacts that define expected contract behavior. - Compatibility checks that detect contract regressions before release. - Fast feedback when output changes violate durability expectations. ## Definition of done - Golden/reference fixtures exist for representative output scenarios. - Compatibility validation is part of normal quality gates. - Contract drift causes deterministic test/gate failures. ## Validation included in this issue - Fixture integrity checks and snapshot stability checks. - Backward-compatibility checks across supported contract versions. - Gate assertions integrated into standard validation flow. ## Acceptance checks 1. Reference fixtures are available and versioned. 2. Compatibility gate runs automatically and blocks regressions. 3. Contract-breaking changes fail validation before release.

redknightlois

Proposed scope alignment from downstream consumer integration: - Add canonical fixture packs for representative artifact variants and contract versions. - Add compatibility gates that validate schema and semantic invariants against fixtures. - Optional follow-up: provide a minimal shared contracts package to reduce duplicated envelope and contract code across satellite repos. This keeps #12 focused on regression prevention and lowering consumer maintenance cost.