The filing_taxonomy_snapshot table was missing parser_engine and related columns
on databases created before the taxonomy surface sidecar migration. This caused
filing sync workflows to fail with 'table has no column named parser_engine'.
Changes:
- Add TAXONOMY_SNAPSHOT_REQUIRED_COLUMNS constant for required columns
- Add verifyCriticalSchema() to fail fast at startup if schema is incompatible
- Reorder ensureTaxonomySnapshotCompat to check table existence before column ops
- Add explicit column verification after ALTER TABLE attempts
- Add regression tests for missing column detection
Fixes#2
