Fix Vermont student loan interest and CTC phase-out, Utah's at-home parent credit and Wisconsin's homestead claimant

[MaxGhenis]

Summary

Four statutory errors in Vermont, Utah and Wisconsin formulas, found while applying those states' rules for a filer who can be claimed as a dependent (#10044). They change results for households where nobody is claimed, so they go to main on their own instead of waiting on that stack.

Formula	Rule now	Source
vt_student_loan_interest_subtraction (new), gov.states.vt.tax.income.agi.subtractions, ...agi.student_loan_interest.income_limit and .in_effect (new)	From 2022: interest paid minus the federal deduction, for AGI up to $200,000 joint or $120,000 otherwise. None in 2021. The model subtracted the raw interest from 2021, a second deduction for anyone who took the federal one, with no income limit	32 V.S.A. 5811(21)(B)(vi), (29)(B): "the amount of interest paid by a qualified resident taxpayer during the taxable year on a qualified education loan", AGI "equal to or less than ... $120,000.00 ... or ... $200,000.00 if ... married filing jointly"; 2022 Act 138, Sec. 5, effective "retroactively on January 1, 2022" (Sec. 13, p. 15); IN-112 line 16 (2025): 16a interest paid, 16b "Student loan interest already deducted on federal Form 1040, Schedule 1, Line 21", 16c the difference. The 2021 IN-112 has no student loan line. (tax.vermont.gov PDFs return 403; NBER archive copies of the department's booklets.)
vt_ctc	$20 per $1,000 over $125,000 comes off each child's $1,000, not off the total	5830f(b): "the amount of the credit per child under this section shall be reduced, but not below zero, by $20.00 for each $1,000.00"
ut_at_home_parent_credit_potential	$100 per qualifying child per return, not per eligible parent	Utah Code 59-10-1005(2): $100 "for each qualifying child if ... the claimant or another claimant filing a joint individual income tax return with the claimant is an at-home parent"
wi_homestead_eligible	One spouse, the claimant, must be 18 or older; 62 or older, disabled or with earned income; and not claimed as a dependent unless 62 or older	Schedule H instructions (2025): Step 1 ("in order to meet this qualification you must be the claimant"), Step 2a "Were you claimed or will you be claimed as a dependent on someone else's 2025 federal income tax return? (If you were age 62 or older ... this limitation does not apply"; p. 10: "Only one spouse must have earned income ... The spouse that meets this definition must be the claimant." Wis. Stat. 71.53(2)(d)

Examples (all 2025 unless noted):

• Vermont, single, wages $10,000, interest $1,000: the federal deduction already takes $1,000, so Vermont AGI is $9,000 (was $8,000). Interest $4,000 at $50,000: Vermont subtracts $1,500. Single AGI $125,000: nothing. 2021: nothing.
• Vermont, two children aged 3, AGI $150,000 (2023): $1,000 (was $1,500).
• Utah, both spouses earning $3,000 or less, one newborn: $100 (was $200); two children: $200 (was $400).
• Wisconsin, couple aged 30 where only the spouse claimed on a parent's return earns: no claimant (was eligible); a claimed claimant aged 62 or older stays eligible.

Readings for Max (queue via the US + core hub)

1. Wisconsin homestead, earned income. Schedule H Step 1a lists "Have earned income" for "you (or your spouse, if married)" without the claimant condition it attaches to the age and disability items; page 10 says the earning spouse "must be the claimant". This PR follows page 10. Household reading: a couple where only the claimed spouse earns would qualify.
2. "Claimed" vs "can be claimed". The homestead bar is an actual claim ("Were you claimed or will you be claimed"), so it reads claimed_as_dependent_on_another_return.

Invariants

1. Vermont: subtraction + federal deduction = filers' qualified interest paid, for AGI within the limit from 2022; zero above the limit and in 2021; never negative.
2. Vermont CTC = (eligible children) x max($1,000 - $20 x increments, 0): linear in children at any AGI.
3. Utah: the credit does not change when a second spouse also qualifies as an at-home parent.
4. Wisconsin: eligibility is invariant to which spouse is labelled head; marking a spouse as claimed never makes a return eligible.

Tests

Variable-named YAML cases for each change (arithmetic in each case's comments); the existing Utah two-parent case and Wisconsin eligibility cases are rewritten on person inputs. Results: pending (local run queued behind the shared heavy lock).

Microsimulation impact

Pending: default dataset, 2025 and 2026, main vs this branch (queued).

axiom: 32 V.S.A. 5811(21)(B)(vi), 5830f(b) queued (draft VT) | Utah Code 59-10-1005 queued (draft UT) | Wis. Stat. 71.53(2)(d) and Schedule H queued (draft WI); dispatch-ready rulespec-us drafts in the claimable-filer set (~/reviews/us-hub/claimable-filer-rules/axiom/s3-ri-sc-ut-vt-wv-wi-drafts.md), held for Max's go, not filed. No module on rulespec-us origin/main encodes these provisions.

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