Read the Michigan home heating credit from the canonical heating inputs

[hua7450]

Stage 2b of the heating-input consolidation (#9284, under #9275), following #9360 and the SNAP half in #9397. Moves the Michigan home heating credit onto the canonical heating inputs. Refs #8964.

What the statute and form say

• MCL 206.527a(1)(c)(i): the standard credit is multiplied by 50% "for a claimant whose heating costs are included in the claimant's rent". The 2025 MI-1040CR-7 book (lines 40–43) adds that a claimant whose "heat cost is included with your rent" may not use the alternate credit.
• MCL 206.527a(11): "Only 1 credit may be claimed for a household", and 206.508(3) defines household as "a claimant and spouse", so the claim unit is the tax unit, not the dwelling.
• Shared housing (Michigan Treasury guidance, "Home Heating Credit and Shared Housing Situations"; CR-7 book p. 5): when two or more single adults share a home, each may claim a credit if each has contracted to pay rent or owns a share.

Changes

Variable	Before	After
heating_expenses (tax unit)	adds = ["heating_expense_person"]	Formula: the SPM unit's heating_expense split evenly across the tax units in that SPM unit (one head per tax unit). UNSPECIFIED households keep the person-level sum as the deprecated adapter.
mi_home_heating_credit_heat_included_in_rent	—	New tax-unit bool, defined_for MI: heat_expense_included_in_rent (SPM unit) or utilities_included_in_rent. Rent covering all utilities necessarily covers heat, so the second term is an implication, not a fallback.
mi_standard_home_heating_credit	halved on utilities_included_in_rent	halved on the new variable
mi_alternate_home_heating_credit_eligible	~utilities_included_in_rent	~ the new variable
utilities_included_in_rent, heating_expense_person	—	documentation updated (Maine PTFC line 5b remains the primary reader of the former)

Behavior preservation

• Every existing MI test passes unchanged except one, which gains state_code: MI because the new variable is state-scoped (its flat inputs had no state before).
• Households that set utilities_included_in_rent: true still get the halved standard credit and no alternate credit, through the implication.
• Households that set heating_expense_person and no heating_type get the same heating_expenses as before.
• Live traffic (household API analytics, 14 days to 2026-09-06): zero requests ask for any MI home heating credit output or send utilities_included_in_rent or heating_expenses, so no partner value moves.

Deliberately not in this PR

• The Reconcile tenant_pays_utilities (Household) and utilities_included_in_rent (TaxUnit): near-inverse vars, different entities, contradictory defaults #8964 default flip (tenant_pays_utilities False → True) and deriving utilities_included_in_rent from it. Michigan's statutory fact is heat in rent, which heat_expense_included_in_rent already carries, so the credit migrates without touching that pair. The flip itself would move ~30 baseline tests plus partner signature and edge-case fixtures that request HUD outputs without setting the flag, so it needs the partner notification gate and its own PR. This satisfies the "MI migrates once" sequencing note on Atomic inputs: consolidate the heating/utility expense input family (LIHEAP, SNAP SUA, CA utility programs) #9284: MI no longer depends on that reconciliation.
• Shared-housing standard allowance division (combine claimants' exemptions, divide the allowance by the number of claimants). A separate rule from the heating-cost split; not modeled before or after this PR.
• Stage 3 (deleting the UNSPECIFIED-gated branches and deprecated inputs) after partner migration.

Tests

• New heating_expenses.yaml (6 cases): legacy person sum, known type reads the fuel bill, two tax units split the bill, known type ignores the legacy input, NONE is zero, per-fuel bill alone is not read.
• New mi_home_heating_credit_heat_included_in_rent.yaml (4 cases) including the implication and the out-of-state case.
• Appended: standard credit heat-in-rent halving, alternate credit heat-in-rent bar, and three integration cases: canonical inputs reproducing test 1 exactly (623 / 853 / 767.70), heat in rent (426.50 / 383.85, alternate 0), and two single adults sharing a $2,000 gas bill (each 1,000 heating cost, 489 standard, 623 alternate, 560.70 credit).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f9b5549) to head (a5e41a3).
⚠️ Report is 52 commits behind head on main.

Additional details and impacted files

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##              main     #9398   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            2         6    +4     
  Lines           20        85   +65     
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+ Hits            20        85   +65     

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