Fix West Virginia low-income exclusion double count

[PavelMakarchuk]

Summary

• remove the low-income earned-income exclusion from West Virginia Schedule M subtractions
• preserve its single application between West Virginia adjusted gross income and taxable income
• add an integrated 2026 regression covering the intermediate values and pre-credit tax

Statutory and form basis

West Virginia IT-140 lines 1-7 compute West Virginia adjusted gross income from federal AGI plus Schedule M additions minus Schedule M subtractions, then subtract the low-income earned-income exclusion and personal exemptions to reach taxable income. This matches West Virginia Code sections 11-21-10 through 11-21-12 and prevents the exclusion from being applied twice.

Checks

• focused wv_taxable_income YAML: 3 passed
• full baseline WV income-tax YAML directory: 109 passed
• WV contrib/reform YAML: 16 passed (test assertions completed; Python 3.14 process was interrupted during teardown)
• household marginal-tax-rate YAML: 5 passed
• make format (including Ruff): passed
• git diff --check: passed
• independent review-fix-review cycle: CLEAN at ad56b65

No partner contract files are changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (4c6ac7e) to head (ad56b65).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #9166   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         1    -2     
  Lines           49        14   -35     
=========================================
- Hits            49        14   -35     

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

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[PavelMakarchuk]

@MaxGhenis This is ready for maintainer review: exact reviewed head ad56b658abc3 is 33/33 green. The independent review-fix-review cycle corrected the public WV AGI structure (single low-income exclusion application) and ended CLEAN; full WV income-tax checks are 109/109.