Fix IA standard deduction to include elderly/blind additional

[PavelMakarchuk]

Summary

Iowa's standard deduction was missing the elderly/blind additional amount. The code used the base federal parameter (fed_p.standard.amount[filing_status] = $31,500 MFJ) instead of the full federal standard_deduction variable ($34,700 for elderly joint couple).

Closes #7936

Root Cause

Both ia_standard_deduction_joint.py and ia_standard_deduction_indiv.py looked up fed_p.standard.amount[filing_status] which returns only the basic standard deduction, omitting:

• Additional standard deduction for elderly/blind ($1,600 per person 65+)

Per IA 1040 line 1d: "Standard deduction or itemized deductions from federal 1040, line 12e" — which includes the full federal standard deduction.

Fix

Replaced the parameter lookup with the standard_deduction variable (= basic_standard_deduction + additional_standard_deduction + bonus_guaranteed_deduction). For the individual (MFS) path, each spouse gets half.

Test plan

• Existing 2021/2022/2023 tests still pass (no elderly, amounts unchanged)
• New 2025 elderly joint test: $31,500 + 2×$1,600 = $34,700
• New 2025 non-elderly joint test: $31,500
• CI passes

References

• Iowa Code §422.7 — "net income means the taxable income as properly computed for federal income tax purposes under section 63"
• IA 1040 line 1d: "Standard deduction from federal 1040, line 12e"

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d95d3cd) to head (99e93f7).
⚠️ Report is 144 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #7937    +/-   ##
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  Coverage   100.00%   100.00%            
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  Files           19         2    -17     
  Lines          297        42   -255     
  Branches         0         2     +2     
==========================================
- Hits           297        42   -255     

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

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