Calibrate ACA take-up contract

[daphnehanse11]

Summary

• derive takes_up_aca_if_eligible from reported Marketplace coverage or a reform-responsive simulated take-up switch
• add simulated_aca_take_up_if_eligible so callers can choose observed-only take-up without losing the existing default simulated behavior
• document selected_marketplace_plan_benchmark_ratio as an explicit user/data input, defaulting to the benchmark-plan assumption when absent
• add tests for observed Marketplace take-up and an unsubsidized reported Marketplace buyer using a selected-plan ratio below the benchmark

Closes #8168.

Tests

• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/aca/takes_up_aca_if_eligible.yaml policyengine_us/tests/policy/baseline/gov/aca/person_receives_aca.yaml policyengine_us/tests/policy/baseline/gov/aca/ptc/marketplace_net_premium.yaml 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/assigned_aca_ptc.yaml -c policyengine_us
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/aca -c policyengine_us
• uv run pytest policyengine_us/tests/test_parameter_files.py -q
• uv run --extra dev ruff check policyengine_us/variables/gov/aca policyengine_us/tests/policy/baseline/gov/aca
• uv run --extra dev ruff format --check policyengine_us/variables/gov/aca/takes_up_aca_if_eligible.py policyengine_us/variables/gov/aca/simulated_aca_take_up_if_eligible.py policyengine_us/variables/gov/aca/ptc/selected_marketplace_plan_benchmark_ratio.py
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (bf8b00a) to head (4a479a0).
⚠️ Report is 35 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #8352       +/-   ##
============================================
+ Coverage   78.70%   100.00%   +21.29%     
============================================
  Files        4750         3     -4747     
  Lines       69270        30    -69240     
  Branches      341         0      -341     
============================================
- Hits        54517        30    -54487     
+ Misses      14675         0    -14675     
+ Partials       78         0       -78     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+21.29%)	⬆️

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