feat(037): Phase 1 fixtures and DTOs (T001-T005)
- T001: Superset dashboard fixture (FI-0080, 3 charts, 2 datasets, 2 filters)
- T002: Result fixtures (scalar, decimal, date, table, locale, malformed)
- T003: Baseline catalog fixtures (valid, invalid_no_release, stale, immutability)
- T004: Materialize fixtures into backend/tests/fixtures/dashboard_testing/
- T005: Pydantic DTOs — 30+ models covering query model, filters, execution,
normalization, comparison, baseline catalog, candidates, structure diff,
verification runs. All extra_forbid, typed, with invariants.
busya
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