[codex] Add clone origin flags and ECPS diagnostics

[MaxGhenis]

Summary

• add explicit clone-origin flags to extended/enhanced CPS artifacts
• add prefix-aware dataset validation for entity-scoped auxiliary variables
• compute and save ECPS clone diagnostics covering clone weight share, modeled-only-poor share, and extreme childcare/tax checks

Testing

• uv run pytest -q tests/unit/test_extended_cps.py tests/unit/calibration/test_calibration_puf_impute.py tests/unit/test_dataset_validation.py tests/unit/test_enhanced_cps_clone_diagnostics.py