Add a tax-only Roth conversion input

[MaxGhenis]

Summary

• add a taxable_roth_conversions input variable for tax-only Roth conversion amounts
• include that variable in IRS gross income without routing it through benefit-facing retirement-distribution income
• add targeted policy tests for IRS gross income, SNAP unearned income, and SSI unearned income

Why

Downstream retirement modeling needs a way to represent direct Roth conversions that affect tax gross income without automatically treating them as spendable retirement-distribution income for means-tested benefits.

This keeps a cleaner separation between:

• taxable Roth conversion amounts used for tax modeling
• retirement distributions used in benefit income logic

That separation matches the legal structure more closely:

• SSI uses its own income definition in 42 U.S.C. 1382a, not IRS gross income
• SSI regulations exclude resource conversions from income in 20 CFR 416.1103(c)
• SNAP uses its own income rules in 7 CFR 273.9; retirement benefits are listed in unearned income under 7 CFR 273.9(b)(2), while nonrecurring lump-sum payments are excluded from income under 7 CFR 273.9(c)(8)

This PR does not try to fully resolve every Roth-conversion treatment question for benefit programs. It just avoids forcing a tax-only Roth conversion input through retirement_distributions.

Verification

• uv run policyengine-core test policyengine_us/tests/policy/baseline/household/income/person/retirement/taxable_roth_conversions.yaml -c policyengine_us
• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/irs/irs_gross_income.yaml -c policyengine_us
• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/ssa/ssi/income/ssi_unearned_income.yaml -c policyengine_us
• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/usda/snap/income/snap_unearned_income.yaml -c policyengine_us
• uv run python -m py_compile policyengine_us/variables/household/income/person/retirement/taxable_roth_conversions.py policyengine_us/tools/default_uprating.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (cc51b69) to head (4abaf29).
⚠️ Report is 2977 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
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  Coverage   100.00%   100.00%           
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  Files            2         1    -1     
  Lines           43         8   -35     
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- Hits            43         8   -35     

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

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