IA pension exclusion applies the $12,000 married cap per spouse instead of as a combined limit

[DTrim99]

Bug

ia_pension_exclusion is a person-level variable that caps each eligible spouse's exclusion at the filing-status maximum:

exclusion_cap = p.maximum_amount[filing_status]  # 12,000 for JOINT
return min_(pension, exclusion_cap)              # per person!

For a married couple where both spouses are 55+ with pension income, each spouse can exclude up to $12,000 — up to $24,000 per couple. The 2021 IA Expanded Instructions (p. 27, the variable's own reference) make the cap a combined limit:

"The exclusion is up to $6,000 for individuals who file status 1, 5, or 6 and up to $12,000 for married taxpayers who file status 2, 3, or 4. ... The exclusion of $8,000 is allowed since a married couple is allowed a combined exclusion of up to $12,000." (Example 2)

The examples also show there is no per-spouse $6,000 sub-cap: a single eligible spouse with $8,000 of pension excludes the full $8,000 against the couple's $12,000 limit, and only eligible (55+/disabled) spouses' pension counts.

Reference: https://revenue.iowa.gov/sites/default/files/2023-01/2021%20Expanded%20Instructions_010323.pdf#page=27

Impact

From PolicyEngine/policyengine-taxsim#1110 (IA 2021, married separate-on-combined, ages 58/57, $104,762 wages, $44,174 pensions split 22,087/22,087, $14,575 interest):

	PolicyEngine	IA 1040 (TaxAct)
Pension exclusion	24,000	12,000 (6,000 + 6,000)
Net income (line 26)	139,511	151,511
Tax before credits	6,919.50	7,750

Applies to any 2021–2022 Iowa couple where both spouses are eligible and combined pensions exceed $12,000. (2023+ the exclusion is unlimited, so the bug is inert.)

Fix

Apply the cap at the tax-unit level and allocate across eligible spouses in proportion to each one's pension income. Validated against the #1110 record: net income 151,511 (exact), tax before credits 7,749.70 vs 7,750, tax after exemption credits 7,669.70 vs 7,670.

Reproduction

TAXSIM input row (reconstructed from the return; the out2psl/1110 data files hold an unrelated AR 2025 record):

taxsimid,year,state,mstat,page,sage,depx,pwages,intrec,pensions,idtl
1110,2021,16,2,58,57,0,104762,14575,44174,5