Add ACA cost-sharing reduction actuarial values

[daphnehanse11]

Summary

• Add a selected_marketplace_plan_category variable defaulting to Silver.
• Use actual lcbp when the selected Marketplace plan category is Bronze, while preserving existing Silver/SLCSP default behavior.
• Add Bronze and Silver standard Marketplace plan actuarial values.
• Add ordinary income-based Silver cost-sharing reduction eligibility, category, CSR actuarial value, and effective actuarial value variables.
• Add tests for the 94%, 87%, and 73% CSR tiers; no CSR at 275% FPL; no ordinary CSR for Bronze; and no direct change to marketplace_net_premium.

Out of scope

• Dollar-valued expected Marketplace MOOP / CSR benefit value.
• AI/AN zero- and limited-cost-sharing plan variations.

Tests

• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/aca/csr policyengine_us/tests/policy/baseline/gov/aca/ptc/selected_marketplace_plan_premium_proxy.yaml policyengine_us/tests/policy/baseline/gov/aca/ptc/used_aca_ptc.yaml policyengine_us/tests/policy/baseline/gov/aca/ptc/marketplace_net_premium.yaml -c policyengine_us
• uv run pytest policyengine_us/tests/test_parameter_files.py
• uv run --extra dev ruff format --check .
• uv run --extra dev ruff check .

Closes #8188.
Closes #8211.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d55cd4f) to head (3fdaad4).
⚠️ Report is 56 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            2         7    +5     
  Lines           35       101   +66     
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+ Hits            35       101   +66     

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

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