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 instead of uprating the input directly

Validation

• PYTHONPATH=. python3 -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 -c policyengine_us
• PYTHONPATH=. pytest -q policyengine_us/tests/policy/baseline/parameters/test_uprating_extensions.py
• git diff --check

Notes

• The weekly work-expense series comes from Census SPM technical documentation and the 2023/2024 poverty reports.
• A raw CPS 2024 check suggests the capped-childcare variable is best interpreted as work_expenses + min(childcare_expenses, max(lower_earner_cap - work_expenses, 0)), not min(work_expenses + childcare_expenses, lower_earner_cap).
• Paired data PR: Populate CPS inputs for SPM childcare formula policyengine-us-data#705.

[MaxGhenis]

Alignment check against the official 2025 ASEC public-use file for 2024 looks strong after the latest childcare-cap fixes.

Direct raw-CPS reconstruction now gives:

• SPM_WKXPNS weighted total ratio: 1.00185
• SPM_CAPWKCCXPNS weighted total ratio: 0.99958
• SPM_CAPWKCCXPNS positive-unit MAE: about $17
• SPM_CAPWKCCXPNS positive-unit share within $1: about 97.22%

So the formula now appears directionally and aggregately aligned to Census. The remaining gap is a narrow unit-level tail, not a broad bias.

Current tail read:

• only about 12% of the top-100 cap misses are cohabiting units
• cohabitors account for only about 9.8% of top-100 absolute cap error
• the remaining misses are mostly in more complex non-cohabiting married/multi-adult SPM units

I filed the follow-up here to track that tail separately: #7961.

My read is that this PR is ship-ready, with the remaining work clearly scoped as a follow-up alignment issue rather than a blocker.