Derive clone-half childcare caps deterministically

[MaxGhenis]

Summary

• stop stage-2 QRF imputation of spm_unit_capped_work_childcare_expenses
• rebuild clone-half capped childcare from clone pre-subsidy expenses, donor cap shares, and clone earnings caps
• add regression tests that lock out impossible capped-childcare values

Testing

• uv run pytest -q tests/unit/test_extended_cps.py tests/unit/calibration/test_calibration_puf_impute.py
• uv run ruff check policyengine_us_data/datasets/cps/extended_cps.py tests/unit/test_extended_cps.py
• uv run ruff format --check policyengine_us_data/datasets/cps/extended_cps.py tests/unit/test_extended_cps.py
• git diff --check