Align Medicare Part B premium modeling with enrollment targets

[daphnehanse11]

Summary

• Add gross_medicare_part_b_premium_if_enrolled for enrollment-consistent CMS premiums-from-enrollees calibration targets.
• Add medicare_irmaa_magi_two_years_prior as an explicit current-year input with fallback to AGI plus tax-exempt interest from two years prior.
• Route Part B and Part D IRMAA through the shared lagged MAGI variable and use the enrollment-gated gross Part B premium in Medicare cost.

Tests

• make format
• uv run pytest policyengine_us/tests/policy/baseline/gov/hhs/medicare/test_part_b_msp_offset.py
• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/hhs/medicare/eligibility/income_adjusted_part_d_premium_surcharge.yaml -c policyengine_us
• uv run pytest policyengine_us/tests/test_system_import.py

Closes #8677

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (47cb26b) to head (f6c02cf).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8697   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            3         5    +2     
  Lines           47        86   +39     
=========================================
+ Hits            47        86   +39     

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

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