Fix unsubsidized Marketplace net premiums

[MaxGhenis]

Summary

• Use pays_aca_premium rather than PTC eligibility for gross SLCSP age-curve and NY/VT family-tier premiums.
• Gate selected_marketplace_plan_premium_proxy on ACA premium payment/takeup instead of aca_ptc > 0, so high-income unsubsidized Marketplace households keep a gross selected-plan premium and net premium.
• Include marketplace_net_premium in household_health_costs when health benefits are included in net income.

Notes

This intentionally does not add Marketplace premiums to SPM MOOP yet; that path needs imputed-premium residual work to avoid broad baseline double counting. This is the household-net-income side of #8095 plus the unsubsidized gross-premium fix.

Tests

• uv run ruff format --check policyengine_us/variables/gov/aca/slspc/slcsp_age_curve_amount_person.py policyengine_us/variables/gov/aca/slspc/slcsp_family_tier_category.py policyengine_us/variables/gov/aca/ptc/selected_marketplace_plan_premium_proxy.py
• uv run ruff check policyengine_us/variables/gov/aca/slspc/slcsp_age_curve_amount_person.py policyengine_us/variables/gov/aca/slspc/slcsp_family_tier_category.py policyengine_us/variables/gov/aca/ptc/selected_marketplace_plan_premium_proxy.py
• uv run python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/gov/aca --batches 1
• uv run python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/household/income/household/household_health_costs.yaml --batches 1
• uv run python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/household/household_health_benefits.yaml --batches 1
• uv run python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/household/household_net_income_including_health_benefits.yaml --batches 1

I also tried uv run python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/household --batches 1; it stayed CPU-bound for about 9 minutes without test output, so I stopped it and used the narrower household tests above.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (20705b0) to head (629248e).
⚠️ Report is 37 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #8163       +/-   ##
============================================
+ Coverage   85.36%   100.00%   +14.63%     
============================================
  Files           3         3               
  Lines          41        60       +19     
  Branches        2         0        -2     
============================================
+ Hits           35        60       +25     
+ Misses          6         0        -6     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+14.63%)	⬆️

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