Route IA CDCC fraction lookup through consolidated taxable income

[PavelMakarchuk]

Summary

Closes #8563.

The Iowa child/dependent care credit fraction lookup compares "taxable income" against bracket thresholds (75 % below $10K, 65 % to $20K, etc.). PE-US's ia_cdcc was reading the dynamically generated ia_taxable_income, which resolves to the legacy ia_taxable_income_joint / _indiv path that starts from ia_net_income (Iowa AGI minus IA deductions). The 2022 IA tax reform (HF 2317) shifted Iowa's tax base to federal taxable income for 2023+, so the credit should compare against ia_taxable_income_consolidated instead.

This also fixes the failure surfaced in PolicyEngine/policyengine-taxsim#953. Federal taxable income for the sample HoH-age-75 household is $7,166 (correctly netting the OBBBA $6K enhanced senior deduction), which lands in the < $10K 75 % bracket. The legacy path was returning $13,166 (missing the senior deduction), landing in the $10K–$20K 65 % bracket, so ia_cdcc was $448.50 instead of TaxAct's $517.50.

Add a formula_2023 branch that switches the comparand for 2023+. Pre-2023 behavior is preserved by leaving the original formula intact (it continues to read the legacy ia_taxable_income).

Test plan

• New integration test from the issue (HoH age 75, $25K wages, $13.8K interest, 1 dep, $3.5K childcare, 2025, IA): ia_taxable_income_consolidated: 7_166, ia_cdcc: 517.5. Numerically matches TaxAct within $1.
• New 2023 paired test: $1K cdcc_potential + $9,999.99 consolidated taxable income → $750 (75 % bracket).
• Existing 2021/2022 unit tests unchanged.
• Full IA suite (241 tests) passes.
• `make format` clean.
• CI green.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (6564ef9) to head (17cda0d).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

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[hua7450]

✅ Approving — verified correct against statute and forms

Reviewed the IA CDCC routing change against the Iowa Code and the official IA 1040 forms (multi-agent review + PDF audit). The fix is correct.

Why the change is right

• Iowa Code §422.12C ties the CDCC percentage to "net income"; §422.7 defines 2023+ "net income" as federal taxable income under IRC §63 ± Iowa modifications — i.e. exactly IA 1040 line 4.
• The 2023 IA 1040 (Step 4) confirms line 4 = line 2 (federal taxable income) + line 3 (net Iowa modifications), which is precisely what ia_taxable_income_consolidated computes. So routing the 2023+ fraction lookup through it is the correct comparand, and the federal Schedule 1-A / OBBBA senior deduction correctly flows into the credit base.
• The 2023 effective boundary aligns with the broader Iowa tax-base switch (the legacy MFS path is already disabled for 2023+). Keeping the legacy ia_taxable_income for pre-2023 preserves prior behavior.
• All 7 fraction brackets (75/65/55/50/40/30/0%) match the 2024 IA 1040 Expanded Instructions p.43 — no parameter change needed.

Test follow-ups pushed (024f9cfe45)

• Tightened the 2025 senior-deduction case's absolute_error_margin from 1 → 0.4 so a wrong fraction bracket can no longer pass silently.
• Added ia_taxable_income: 13_166 so the case now enforces (rather than just comments) that the consolidated income ($7,166 → 75% bracket) and the legacy income ($13,166 → 65% bracket) land in different brackets.
• Merged current main into the branch (was 4 behind).

One thing to watch: the Full Suite - Contrib (other-shard-1) job was failing on the prior head, in a shard this PR doesn't touch — it's now re-running against current main; worth confirming it goes green before merge.