Indiana EITC childless filers still use the frozen 2023 federal EITC (Schedule IN-EIC Section A applies to all filers)

[MaxGhenis]

Summary

#8875 routed Indiana with-children filers to the current-year federal EITC, fixing the frozen-2023 understatement. Childless filers still use the frozen 2023-dollar snapshot in in_eitc.py, which understates the IN EITC by roughly $1–11 per childless filer for TY2024–2025 (10% of the frozen $600 max vs the indexed $632/$649).

The retained split rests on a rationale that does not hold up against the primary sources:

• 2025 Schedule IN-EIC (State Form 49469, R25/9-25) computes the credit in Section A for all filers: Line A-1 is "the earned income credit from your federal income tax return," multiplied by 10%. Section B is a qualifying-child information schedule, not a computation.
• DOR Information Bulletin Add SchXYZTax #92 describes the IRC §32 January 1, 2023 conformity freeze uniformly, with no childless carve-out.
• The ARPA childless expansion expired January 1, 2022, so post-2023 federal childless changes are inflation indexing only — the same indexing Use current-year federal EITC for Indiana EITC with-children filers #8875 accepted for with-children filers.

Suggested fix

Drop the where(child_count > 0, ...) split in policyengine_us/variables/gov/states/in/tax/income/credits/earned_income_credit/in_eitc.py and use the current-year federal EITC for all filers, updating the childless test expectations (2025 childless: $60.00 → $64.90) and correcting the Section B / ARPA rationale in the code comments.

Follow-up to #8875 / #8831.