Add Census SPM work expense formulas

[MaxGhenis]

Summary

• add Census SPM weekly work-expense history as a parameter series with primary-source metadata
• add weeks_worked, spm_unit_work_expenses, and spm_unit_head_spouse_earned_cap
• compute spm_unit_capped_work_childcare_expenses from the model formula as combined work plus childcare expenses capped by the lower earner's earnings

Validation

• PYTHONPATH=. uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/household/income/person/weeks_worked.yaml policyengine_us/tests/policy/baseline/household/expense/childcare/spm_unit_work_expenses.yaml policyengine_us/tests/policy/baseline/household/expense/childcare/spm_unit_capped_work_childcare_expenses.yaml policyengine_us/tests/policy/baseline/household/expense/childcare/spm_unit_head_spouse_earned_cap.yaml -c policyengine_us
• PYTHONPATH=. uv run pytest -q policyengine_us/tests/policy/baseline/parameters/test_uprating_extensions.py
• uv run ruff format --check policyengine_us/variables/household/expense/childcare/spm_unit_capped_work_childcare_expenses.py policyengine_us/variables/household/expense/childcare/spm_unit_work_expenses.py policyengine_us/variables/household/expense/childcare/spm_work_childcare_earnings.py
• uv run ruff check policyengine_us/variables/household/expense/childcare/spm_unit_capped_work_childcare_expenses.py policyengine_us/variables/household/expense/childcare/spm_unit_work_expenses.py policyengine_us/variables/household/expense/childcare/spm_work_childcare_earnings.py
• git diff --check

Notes

• This revives and supersedes fork PR Add Census SPM work expense formulas #7960 on a same-repository branch so CI can run.
• The weekly work-expense series comes from Census SPM technical documentation and the 2023/2024 poverty reports.
• The capped work and childcare expense formula is implemented as the combined work-plus-childcare amount capped by the lower earner's earnings.
• Paired data PR: Populate CPS inputs for SPM childcare formula policyengine-us-data#705.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (623a65c) to head (ea9f29d).
⚠️ Report is 140 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%            
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  Files           12         6     -6     
  Lines          173        73   -100     
  Branches         4         0     -4     
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- Hits           173        73   -100     

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unittests	100.00% <100.00%> (ø)

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