Fix SPM childcare cap projection

[MaxGhenis]

Summary

Fixes the SPM childcare earnings cap formula introduced in #8247 so SPM-unit-level any() results are projected back to person-level before combining them with person-level partner flags. Without this, projected-year microsimulations over datasets with multiple SPM units can fail with a shape/broadcasting error.

Testing

• PYTHONPATH=. uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/household/expense/childcare/spm_unit_head_spouse_earned_cap.yaml -c policyengine_us
• uv run ruff check policyengine_us/variables/household/expense/childcare/spm_unit_head_spouse_earned_cap.py
• uv run ruff format --check policyengine_us/variables/household/expense/childcare/spm_unit_head_spouse_earned_cap.py
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (623a65c) to head (c9a7305).
⚠️ Report is 143 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #8250    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           12         1    -11     
  Lines          173        25   -148     
  Branches         4         0     -4     
==========================================
- Hits           173        25   -148     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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