Apply ACA PTC formula to historical premiums

[daphnehanse11]

Summary

• apply the ACA PTC formula starting in 2018, the first year with rating-area SLCSP premium data
• keep pre-2018 ACA PTC at zero until older rating-area premium data is added
• add unit-style and integration regression coverage for 2023 ACA PTC calculations

Fixes #8527.

Tests

uv run python -m policyengine_core.scripts.policyengine_command test -c policyengine_us policyengine_us/tests/policy/baseline/gov/aca/ptc/aca_ptc.yaml policyengine_us/tests/policy/baseline/gov/aca/ptc/integration.yaml

Also run in the original workspace:

uv run python -m policyengine_core.scripts.policyengine_command test -c policyengine_us policyengine_us/tests/policy/baseline/gov/aca

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ee131a9) to head (dda8d96).
⚠️ Report is 33 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #8531    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files            9         1     -8     
  Lines          149        17   -132     
==========================================
- Hits           149        17   -132     

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

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.