Give the Illinois 2021 income tax rebate base amount at negative AGI

[PavelMakarchuk]

Fixes #9821.

Change

il_income_tax_rebate floors federal AGI at 0 before the amount.joint / amount.other scale lookup. Section 212.1(a) of P.A. 102-0700 has only an upper AGI limit ($400,000 joint, $200,000 other), so a filer with a negative AGI gets the $50 / $100 base amount. Eligibility (il_income_tax_rebate_eligible) does not change.

Tests

Three new 2021 cases. The expected values are TAXSIM-35 srebate outputs for the same filers:

Test	Expected	Before
Single, AGI −2,000	50	0
Single, AGI −2,000, 1 dependent	150	100
Joint, AGI −500,000, 2 dependents	300	200

All 898 Illinois tests pass locally.

Census effect

In the TAXSIM-35 census comparison for Illinois, 8 household-years change and none that matched before stop matching. 5 now match TAXSIM. The other 3 also need #9820 (state EITC helper); with both changes they match.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (909176a) to head (da3c7ac).
⚠️ Report is 202 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            4         1    -3     
  Lines           76        19   -57     
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- Hits            76        19   -57     

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

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[hua7450]

Approving. Reviewed against Public Act 102-0700, Sec. 212.1 (pages 131 to 134 of the ILGA PDF) and the IDOR rebate page (Wayback, 2023-01-01).

Subsection (a) is the only income test in the section and it is an upper bound: "whose adjusted gross income for the taxable year is less than (i) $400,000, in the case of spouses filing a joint federal tax return, or (ii) $200,000, in the case of all other taxpayers, is entitled to a one-time rebate". Subsections (b) to (i) add no second test, and (f) confirms EITC recipients also qualify. IDOR's page says the same and tells filers with no federal filing requirement to file with "0" on Line 1 to claim it, so a negative AGI getting the base amount is right.

The three new cases recompute by hand: floored AGI 0 falls in the first band ($50 / $100), plus $100 x min(dependents, 3), giving 50, 150 and 300 (main returns 0, 100, 200). The $200,000 / $400,000 boundaries are unchanged: exactly at the limit gives 0, one dollar below gives the full base. Entity, period, vectorization and the downstream il_refundable_credits list (2021 only) all check out.

Optional, no action needed: a -.inf first threshold in the two scale parameters would express the same rule declaratively instead of flooring in the formula.

The two attempt-1 CI failures (Contrib states-shard-2 and other-shard-2a) were runner shutdowns ("The runner has received a shutdown signal", exit 143), not assertions; attempt 2 on the same commit passed.

Not verified: TAXSIM-35 outputs, the census comparison counts, and the local test run.