Apply IA pension exclusion married cap as a combined limit

[DTrim99]

Fixes #9155

Surfaced by PolicyEngine/policyengine-taxsim#1110 (IA 2021, married separate-on-combined, ages 58/57, $44,174 of pensions split evenly between spouses).

Changes

The 2021 IA Expanded Instructions (p. 27, the variable's own reference) make the married pension-exclusion cap a combined limit: "a married couple is allowed a combined exclusion of up to $12,000" (Example 2), limited to each eligible (55+/disabled) spouse's own pension, with no per-spouse $6,000 sub-cap.

ia_pension_exclusion previously applied min(pension, maximum_amount[filing_status]) per person, letting each eligible spouse exclude up to $12,000 — $24,000 per couple. The formula now caps the tax-unit total of eligible pension at the filing-status maximum and prorates it across eligible spouses by pension share.

Inert for 2023+ (the cap is .inf). No parameter changes.

Validation

Against the #1110 record (IA 1040: exclusion 6,000 + 6,000, net income 151,511, tax before credits 7,154 + 596 = 7,750, tax after exemption credits 7,670):

	Before	After	IA 1040
Pension exclusion	24,000	12,000	12,000
Net income	139,511	151,511	151,511
Tax before credits	6,919.50	7,749.70	7,750
Tax after exemption credits	6,839.50	7,669.70	7,670

New tests: the #1110 both-spouses-over-cap case ([6,000, 6,000]), Example 2 from the instructions (single eligible spouse excludes the full $8,000), and uneven proration ([9,000, 3,000] on 30k/10k). All 10 tests in the file pass locally, including the 7 pre-existing ones.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (db70094) to head (36713ab).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

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##              main     #9156    +/-   ##
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  Coverage   100.00%   100.00%            
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  Files           17         1    -16     
  Lines          279        20   -259     
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- Hits           279        20   -259     

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