[leoafarias]

Summary

• Pin the complete upstream Draft 2020-12 test corpus and offline remote documents, with license and commit provenance, independently of the importer and model generator in feat(generator): generate models from Draft 2020-12 schemas #167.
• Add a fixture-integrity test that parses all 159 JSON assets and verifies the 46 required files contain 384 groups and 1,301 instance cases. This PR changes no runtime or generator source.

Review order

This is the base of a review stack: #169 → #167 → #168. The fixture files are byte-identical across the branches, and #167/#168 now exclude them from their own diffs. Merge in that order, retargeting each successor to main after its base merges. Executable Ack validation/export and generated-model conformance tests remain in #167: they require its implementation and cannot pass against current main independently.

Validation

• dart run melos run analyze --no-select — passed.
• dart test in packages/ack — 1,311 tests passed, including fixture integrity.
• dart test test/integration/model_generation_guide_build_test.dart in packages/ack_generator — passed.
• git diff --check — passed.