Fix Vermont Act 71 retirement exemption stacking

[saisharan0103]

Issue #9146

Vermont Act No. 71 changed 32 V.S.A. § 5830e(e) so the military retirement and survivor benefit exclusion under subsection (d) may be taken in addition to one elected exclusion under subsections (a)-(c). The existing Vermont retirement income exemption formulas still treated post-2025 exclusions as a single max_() choice among Social Security, military retirement, and CSRS/other retirement amounts, then applied the phase-out band selected from that raw winner.

This updates the 2025+ behavior, keyed by p.military_retirement.income_based_structure.in_effect, to calculate the military income-based exemption separately and add it to the best post-phase-out exclusion among Social Security and CSRS/other contributory retirement. Earlier years keep the existing single-election behavior through the same parameter branch.

Eligibility is also updated so the non-military AGI gate applies only to the elected § 5830e(a)-(c) component, while military income can independently keep the variable defined. Nearby Act No. 71 citation comments are corrected.

Modified files:

• policyengine_us/variables/gov/states/vt/tax/income/adjusted_gross_income/subtractions/retirement_income_exemption/vt_retirement_income_exemption.py
• policyengine_us/variables/gov/states/vt/tax/income/adjusted_gross_income/subtractions/retirement_income_exemption/vt_retirement_income_exemption_eligible.py
• policyengine_us/tests/policy/baseline/gov/states/vt/tax/income/agi/subtractions/vt_retirement_income_exemption.yaml
• policyengine_us/tests/policy/baseline/gov/states/vt/tax/income/agi/subtractions/vt_retirement_income_exemption_eligible.yaml

Diff size: +184 / -27.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (838be3f) to head (6fd1ef4).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            2         2           
  Lines           31        89   +58     
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+ Hits            31        89   +58     

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unittests	100.00% <100.00%> (ø)

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[MaxGhenis]

Cross-family dual review (Claude Fable 5 + GPT-5.6-sol, independent). The core direction is right and verified: for Act 71 years the formula now returns the self-phased military exclusion plus the best post-phase-out election among the modeled (a)/(b) options — exactly the correction #9146 asked for — and the changed arithmetic reproduces by hand (e.g. military $30,000 + SS $1,000 × 0.50 at AGI $60,000 = $30,500; live sims confirm no pre-2025 behavior change). Blockers before this can close #9146:

1. Subsection (c) is still unmodeled. The election in the statute spans (a) Social Security, (b) CSRS, and (c) other-contributory retirement systems; vt_csrs_retirement_pay_exclusion sums only csrs_retirement_pay (documented as federal CSRS only), and no Vermont variable reads any other contributory-pension input. A (c) pension can't qualify or stack. Implement, or rescope with a follow-up issue.
2. Tests mostly don't pin the corrections. The military+SS stacking case is a real regression test, but the high-AGI cases pass under the old code too, and the "larger post-phase-out" case phases both options by the same 0.50 factor so raw-vs-phased election is indistinguishable. Please add a military+CSRS stacking case and an election case where raw and post-phase-out orderings differ.
3. Survivor-benefit convention (issue item 2) is unaddressed. The exclusion reads only military_retirement_pay; a household populated via military_retirement_pay_survivors (used by MD/MN/SC) gets no (d) exclusion. Document or wire the intended convention.
4. Citation cleanup incomplete. csrs/reduction/start.yaml and end.yaml still say "Act No. 51 (2025)" (it's Act 71; Act 51 is S.59), one test comment still says "pre-Act 51", and both changed variable files have a missing comma in the reference tuple, concatenating the statute and IN-112 URLs into one invalid string.