Add safe import checkpoints and invariant validation

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What this task asks for

Add safe bulk import checkpoints, invariant validation, and rollback-on-failure behavior. Create safe-import checkpoints that restore data and schema changes exactly on failure. Support nested checkpoint creation, commit, rollback, cleanup, and distinct inactive or unknown checkpoint errors. Persist table invariants and evaluate SELECT, aggregate, or row-level expressions with structured failure details. Add safe bulk insert, upsert, CSV import, and JSON import operations with strict and non-strict behavior. Expose safe-import and invariant management commands plus safe-mode options for insert, upsert, and bulk. Exit successfully only when safe-mode writes commit while invariant validation commands always exit zero.