Allocate taxable UC by recipient, not all to head

[PavelMakarchuk]

Closes #8494.

Summary

taxable_unemployment_compensation (Person, YEAR) was assigning the tax unit's entire taxable UI to the head and zero to the spouse via where(is_tax_unit_head, tax_unit_taxable_uc, 0). The original shortcut preserved the federal tax-unit AGI sum but broke per-person AGI used by state combined-return optimization (KY, AR, DE, IA, MS, MT, WV, MI). Inflating or zeroing one spouse's apparent AGI shifted state std deductions to the wrong person and produced incorrect state liability.

Fix

Mirror the existing taxable_social_security pattern (taxable_social_security.py:11-29): allocate tax_unit_taxable_unemployment_compensation across persons in proportion to each person's unemployment_compensation. The federal sum is preserved (shares sum to 1); per-person AGI now reflects the actual recipient.

def formula(person, period, parameters):
    tax_unit_taxable_uc = person.tax_unit("tax_unit_taxable_unemployment_compensation", period)
    person_uc = person("unemployment_compensation", period)
    tax_unit_uc = person.tax_unit.sum(person_uc)
    share = where(tax_unit_uc > 0, person_uc / tax_unit_uc, 0)
    return tax_unit_taxable_uc * share

Tests

• New taxable_unemployment_compensation.yaml with 3 cases (spouse-only UC, proportional split, zero UC).
• Existing test suite passes: 299 IRS taxable income tests + 369 KY tests + smoke-tested AR/DE/IA/MS/MT/WV/MI (~1,930 state tests total) — no regressions.

Open question

Running the issue's repro (KY 2025 joint, head $1.13 UC, spouse $10K wages + $19,124 UC) produces ky_income_tax = $994 after the fix, vs. the issue's expected ~$524 (TaxAct). The variable-level fix is correct (per-person UC matches reality), but the KY-side discrepancy needs separate investigation — possibly a different KY combined-return optimization issue, family-size credit, or TaxAct's own handling of the std deduction split. Flagging for KY-specific follow-up before closing the original taxsim issue.

Test plan

• New YAML tests pass
• No regressions in IRS taxable income or KY/AR/DE/IA/MS/MT/WV/MI states
• CI passes
• Validate KY $524 vs TaxAct in follow-up

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (92fdab8) to head (964df60).
⚠️ Report is 100 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8498   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            2         1    -1     
  Lines           57        13   -44     
  Branches         1         0    -1     
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- Hits            57        13   -44     

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

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