Fix VA spouse tax adjustment by calculating separate VAGI per person

[policyengine]

Summary

Fixes #6958

The Virginia Spouse Tax Adjustment incorrectly calculated eligibility because va_agi_person prorated combined VA AGI by Federal AGI share, spreading deductions like the age deduction across both spouses instead of applying them to the correct person.

Changes

1. Created va_age_deduction_person: Calculates age deduction per person based on their individual Federal AGI and age eligibility
2. Created va_additions_person: Person-level Virginia additions (currently returns 0, ready for future additions)
3. Created va_subtractions_person: Person-level Virginia subtractions (currently only age deduction)
4. Fixed va_agi_person: Now calculates separate VAGI for each person from their individual components instead of prorating combined VA AGI
5. Added integration test: Tests the elderly head with low income case from policyengine-taxsim Separate Additional Medicare Tax parameters by payroll / self-employment #664

Test Results

• New integration test passes: VA Spouse Tax Adjustment correctly returns $0 for elderly head with low income
• All 12 existing spouse tax adjustment tests pass
• All 15 age deduction tests pass
• Code formatted with Black

Technical Details

This matches the VA Form 760 Spouse Tax Adjustment Worksheet (page 12) which explicitly shows how to calculate "Separate Virginia Adjusted Gross Income" for each spouse:

• Start with each person's Federal AGI
• Apply person-specific additions
• Apply person-specific subtractions (like age deduction)

The fix ensures that when a 75-year-old head with low income qualifies for a $12,000 age deduction, it only reduces their VAGI (potentially to negative), not the spouse's VAGI.

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ba52386) to head (4a8e8b3).
⚠️ Report is 42 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #6960   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         4    +2     
  Lines           26        58   +32     
=========================================
+ Hits            26        58   +32     

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

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[MaxGhenis]

@PolicyEngine it's failing tests

[policyengine]

I ran into an issue:

Claude Code failed (exit 1): Error: spawn ps ENOENT
    at ChildProcess._handle.onexit (node:internal/child_process:285:19)
    at onErrorNT (node:internal/child_process:483:16)
    at process.processTicksAndRejections (node:internal/process/task_queues:82:21) {
  errno: -2,
  code: 'ENOENT',
  syscall: 'spawn ps',
  path: 'ps',
  spawnargs: [ '-o', 'pid', '--no-headers', '--ppid', 12267 ]
}

[PavelMakarchuk]

@DTrim99 please review, this PR is not a state ready to merge