Fix Vermont retirement-income exemption eligibility gate for Social Security filers

[PavelMakarchuk]

Summary

The Vermont retirement-income exemption eligibility variable vt_retirement_income_exemption_eligible tested AGI against the CSRS reduction-end threshold for all filers, whereas the main exemption formula vt_retirement_income_exemption correctly branches on use_ss to select the Social Security phase-out endpoint. The two were consistent only while the CSRS and Social Security thresholds were identical.

S.51 (2025) (32 V.S.A. § 5830e) raised the Social Security thresholds by $5,000 (single: $60k → $65k eligibility end; joint: $75k → $80k), but the CSRS thresholds were left unchanged. As a result, a Social Security filer with AGI between the stale CSRS end and the true Social Security end was gated out of eligibility and received a $0 exemption, even though the phase-out formula would have granted a partial exemption.

Fix

Mirror the use_ss branch used by the main formula inside the eligibility gate, so both use the same Social Security reduction-end threshold for Social Security electors. This keeps the gate and the formula consistent and preserves the code's existing model of the two elections' thresholds as independent parameter paths (no YAML changes needed).

Evidence

• 32 V.S.A. § 5830e(a)(1)(C), (a)(2)(C)
• S.51 (2025), Vermont tax credit package (raises Social Security thresholds by $5,000)
• 2025 Vermont Income Tax Return Booklet, p. 16 — Retirement Income Exemption Worksheet (Schedule IN-112, Part I, Line 12)

Reproduction (taxsim #999, single, age 74, TY2025)

AGI $60,942 sits in the 2025 single partial band ($55k–$65k). Correct SS end = $65,000 → ratio 0.41 → exemption = $20,725.08 × 0.41 = $8,497.28 (matches filled IN-112 Line 12 = $8,497). Before this fix PE used the stale CSRS end ($60,000), gated the filer out, and returned $0. After the fix PE produces exemption $8,497.28, VT income tax $1,281.20 — matching TaxAct and the worksheet.

Tests

Adds two integration tests to vt_retirement_income_exemption.yaml covering the single (taxsim #999) and joint (taxsim #1000) records. Both pass; neighboring VT tests still pass.

Resolves policyengine-taxsim #999 and #1000.

Fixes #8837

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f9e58e7) to head (5b654ce).
⚠️ Report is 44 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8853   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            3         1    -2     
  Lines           55        24   -31     
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- Hits            55        24   -31     

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

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