Add Wisconsin retirement income exclusion (Line 16, $24K, age 67+)

[PavelMakarchuk]

Summary

• Implements the new Wisconsin retirement income exclusion from Wis. Stat. 71.05(6)(b)54m.a (Act 15, effective tax year 2025)
• Eligible filers age 67+ can exclude up to $24,000 ($48,000 joint with both spouses 67+) of pension and IRA income from Schedule SB Line 16
• Claiming the exclusion forfeits all tax credits, so the implementation computes tax under both paths and picks the lower amount
• Includes 18 YAML tests covering eligibility, exclusion amounts, tax reduction logic, and integration with wi_income_tax

New files

Type	File	Description
Parameter	exclusion/max_amount/single.yaml	$24,000 per-individual cap
Parameter	exclusion/max_amount/joint.yaml	$48,000 joint pooled cap
Parameter	exclusion/min_age.yaml	Age 67 minimum
Variable	wi_retirement_income_exclusion_eligible.py	Age eligibility (head/spouse only)
Variable	wi_retirement_income_exclusion_amount.py	Line 16 amount (pension + IRA, capped)
Variable	wi_retirement_income_exclusion_tax_reduction.py	Two-path tax comparison
Modified	wi_income_tax.py	Applies exclusion reduction before refundable credits

How it works

Path A (standard):  tax_before_refundable - refundable_credits
Path B (exclusion): rates(taxable_income - Line16 + Line17), zero credits
Tax = min(Path A, Path B) if eligible

Test plan

• 4 eligibility tests (age 67/68/66, dependent exclusion)
• 6 amount tests (below/at/above cap, joint both/one eligible, zero pension)
• 5 tax reduction tests (beneficial, not beneficial, ineligible age, Line 17 add-back, joint)
• 3 integration tests on wi_income_tax (exclusion wins, credits win, ineligible)
• Existing WI integration test (2024, pre-Act 15) still passes
• make format passes

References

• Wis. Stat. 71.05(6)(b)54m.a
• 2025 Schedule SB Instructions, p.7

Closes #7425

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0f779b4) to head (51f3fee).
⚠️ Report is 43 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #7426   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         4    +3     
  Lines           22        76   +54     
=========================================
+ Hits            22        76   +54     

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

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.