Fix Virginia age deduction: protect fixed pre-1939 amount from income phase-out

[DTrim99]

Fixes #9302.

What

Virginia grants two distinct age deductions under Va. Code § 58.1-322.03(5):

(a) A deduction in the amount of $12,000 for individuals born on or before January 1, 1939.
(b) A deduction in the amount of $12,000 for individuals born after January 1, 1939, who have attained the age of 65. This deduction shall be reduced by $1 for every $1 that the taxpayer's adjusted federal adjusted gross income exceeds $50,000 for single taxpayers or $75,000 for married taxpayers.

The income test appears only in subparagraph (b). The (a) fixed deduction is never reduced.

va_age_deduction previously computed a single combined maximum ($12,000 × count_eligible) and then subtracted the entire AFAGI excess from the whole thing whenever not every eligible filer qualified for the fixed amount. For a mixed-age couple — one spouse born on/before Jan 1, 1939 (fixed) and one born after (income-based) — this wiped out the protected fixed $12,000.

This PR splits the deduction into the fixed portion (never reduced) and the income-based portion (reduced by the excess), so only the (b) amount is means-tested.

Impact

Only the mixed-age married case changes. All-fixed, all-income-based, and single-filer cases are unchanged.

Failing case (taxsim #1140)

Joint filers, TY2022: taxpayer born 1/1/1937 (fixed $12,000), spouse born 1/1/1945 (income-based), AFAGI $160,294 ≫ $75,000 married threshold so the (b) amount phases to $0.

• TaxAct Form 760: age deduction $12,000, VA taxable income 131,057.
• PolicyEngine before: $0, VA taxable income 143,058 (tax before credits $690 too high).
• PolicyEngine after: $12,000. ✓

Tests

• Corrected the existing joint mixed-age test, which encoded the buggy value ($9,000) — the statute gives $12,000.
• Added the Reduce MD CTC by the federal CTC #1140 record as a regression test.
• All 18 va_age_deduction tests pass.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8326a14) to head (4c42ff5).
⚠️ Report is 55 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%            
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  Files           12         1    -11     
  Lines          161        30   -131     
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- Hits           161        30   -131     

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

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