Apply KY personal credits per column under combined-separate election (#9010)

[PavelMakarchuk]

Fixes #9010.

Problem

For married couples electing filing status 2 (married filing separately on a combined return), PolicyEngine pooled the Schedule ITC Section B personal credits ($40 aged / $40 blind / $20 National Guard) at the tax-unit level and capped them against unit-level tax. Form 740 computes lines 16–18 per column: each spouse's own credit offsets only that spouse's own tax, with a per-column zero floor (line 18: "If line 17 is larger than line 16, enter zero"). PolicyEngine therefore let a low-income elderly spouse's credit offset the other spouse's tax.

On the taxsim #1076 record, TaxAct and TAXSIM-35 both report KY income tax $1,353.50–$1,354; PolicyEngine reported $1,313.50 — the 66-year-old spouse's $40 over-65 credit was pooled into the head's column instead of dying in her near-zero column.

Fix

ky_personal_tax_credits_potential now caps each person's ky_personal_tax_credits_indiv at their own ky_income_tax_before_non_refundable_credits_indiv before summing, under the separate election. Same class as the DE Filing Status 4 fix (#8978).

Scope note (follow-up)

The issue also notes a secondary item: ky_files_separately elects on pre-credit column tax, whereas Form 740's election should compare post-credit column liabilities (line 26). It does not flip this record, and implementing it naively creates a circular dependency (ky_files_separately → post-credit tax → personal credits → ky_files_separately), so it's left as a documented follow-up rather than folded into this fix.

Tests

• Unit: added a per-column floor case (person1 credit $40 capped at $20 column tax; person2 credit $100 capped at $0 → total $20). Updated the existing "separate" case to supply per-column tax so its $140 result still holds.
• Integration: added the taxsim Add TANF to school meals categorical eligibility #1076 case → ky_files_separately: true, ky_income_tax $1,353.50 (matches TaxAct and TAXSIM-35).
• Full KY income tax suite (149 tests) passes.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ff39625) to head (f5e2077).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

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  Files            1         1           
  Lines           18        17    -1     
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- Hits            18        17    -1     

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unittests	100.00% <100.00%> (ø)

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