Testing and final validation conventions

Use model-based tests for stateful systems, property-based tests for parsing, validation, and transformations, and example tests for exact-format, integration, timing-specific, or trivial behavior. Property/model suites use DEFAULT_NUM_RUNS (50); database properties use createIsolatedDbContext() with cleanup in finally. Add deterministic targeted regressions for behavior changes and avoid duplicating invariants already covered by properties. If a contract intentionally changes, update stale mocks and assertions, rerun focused tests, then validate the exact final tree with required generation, typecheck, lint, policy/dependency checks, and the full unit suite. Inspect the final diff and separate unrelated warnings or infrastructure failures from regressions.