Add marketplace selected-plan PTC proxy variables

[daphnehanse11]

Closes #7801

Summary

• add an annual TaxUnit input for selected_marketplace_plan_benchmark_ratio with a default of 1.0
• add derived ACA variables for the selected-plan premium proxy and used versus unused ACA PTC without changing aca_ptc semantics
• add YAML coverage for the benchmark-default, bronze-like ratio, and zero-PTC cases

Testing

• ruff format .
• ruff check .
• python -m py_compile policyengine_us/variables/gov/aca/ptc/selected_marketplace_plan_benchmark_ratio.py policyengine_us/variables/gov/aca/ptc/selected_marketplace_plan_premium_proxy.py policyengine_us/variables/gov/aca/ptc/used_aca_ptc.py policyengine_us/variables/gov/aca/ptc/unused_aca_ptc.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (752fae4) to head (5c81898).
⚠️ Report is 116 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #7802   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            2         4    +2     
  Lines           13        36   +23     
=========================================
+ Hits            13        36   +23     

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

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