Add external CTC child benchmarks to national validation

[MaxGhenis]

Summary

• add an exact 2022 IRS Publication 4801 benchmark for ctc_qualifying_children
• add contextual public IRS benchmark comparisons for qualifying-child shares by AGI band and filing status
• keep these external comparisons clearly separate from hard current-law pass/fail references
• add focused unit coverage and a changelog fragment

Why

Recent CTC follow-up work showed that the remaining instability is compositional. We now have a better validator for artifact-to-artifact drift in #731, but we still need some source-backed external anchors for the child composition itself.

This PR adds the public-source pieces that look defensible now:

• exact total ctc_qualifying_children from 2022 Publication 4801 Schedule 8812 line 4 amounts
• contextual 2021 advance CTC admin-share comparisons by AGI band and filing status

It deliberately does not add the 2021 under-6 split as a hard target because that source is tied to the ARPA/advance-CTC under-18 concept rather than the current-law under-17 concept used by ctc_qualifying_child.

Issue tracking

• Progress on Source external benchmarks for CTC child-composition calibration #729
• Related blocked sourcing issue: Add AGI-by-filing-status CTC targets for refundable and nonrefundable components #717

Testing

• uv run pytest tests/unit/calibration/test_validate_national_h5.py -q
• uv run ruff check policyengine_us_data/calibration/validate_national_h5.py tests/unit/calibration/test_validate_national_h5.py
• uv run ruff format --check policyengine_us_data/calibration/validate_national_h5.py tests/unit/calibration/test_validate_national_h5.py
• uv run python -m policyengine_us_data.calibration.validate_national_h5 --help

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Project	Deployment	Actions	Updated (UTC)
pipeline-diagrams	Error		Apr 12, 2026 0:49am