Add Medicare Part D IRMAA surcharge variable

[MaxGhenis]

Summary

• add Medicare Part D IRMAA parameter ladders for all modeled filing statuses
• add an annualized income_adjusted_part_d_premium_surcharge variable using two-year-lagged MAGI
• add baseline policy tests covering no-surcharge and surcharge cases

Scope

This PR models the federally defined Part D IRMAA surcharge only.

It does not attempt to model a total Part D premium, because underlying Part D plan premiums vary by plan and are not nationally fixed.

Legal basis

• 42 U.S.C. § 1395w-113(a)(7) provides the Part D income-related increase in the beneficiary premium and points to the section 1839(i) threshold / MAGI framework in 42 U.S.C. § 1395r(i).
• 42 CFR § 423.286(d)(4) says the Part D income-related monthly adjustment amount is paid in addition to the Part D premium.
• 20 CFR Part 418, Subpart C contains SSA's determination and collection rules for the income-related monthly adjustment amount.

Annual values source

The 2024 / 2025 bracket thresholds and surcharge amounts in this PR come from CMS fact sheets:

• https://www.cms.gov/newsroom/fact-sheets/2024-medicare-parts-b-premiums-and-deductibles
• https://www.cms.gov/newsroom/fact-sheets/2025-medicare-parts-b-premiums-and-deductibles

Testing

• uv run python -m py_compile policyengine_us/variables/gov/hhs/medicare/eligibility/part_d/income_adjusted_part_d_premium_surcharge.py
• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/hhs/medicare/eligibility/income_adjusted_part_d_premium_surcharge.yaml
• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/hhs/medicare/eligibility/income_adjusted_part_b_premium.yaml
• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/hhs/medicare/costs/medicare_cost.yaml

Closes #7949.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (cc51b69) to head (3676651).
⚠️ Report is 2977 commits behind head on main.

Additional details and impacted files

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  Files            2         1    -1     
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- Hits            43        23   -20     

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

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