Implement Maine PTFC unknown-utilities 15% rule (Schedule PTFC line 5c)

[DTrim99]

Surfaced by PolicyEngine/policyengine-taxsim#1126 (ME renter with rent that includes utilities).

Problem

Maine's property tax fairness credit treats 15% of rent as "rent constituting property taxes," after first excluding any heat/utilities included in the rent. Per 2022 Schedule PTFC/STFC line 5c: if the rent includes utilities and the amount is known, subtract it; if the rent includes utilities and the amount is not known, subtract 15% of gross rent instead.

me_property_tax_fairness_credit_countable_rent only handled the known-amount case (utilities_included_in_rent * utility_expense) and its comment noted the unknown-amount branch was "not implemented." So a filer whose rent includes utilities but who did not itemize a utility amount received no utility exclusion at all, overstating the countable rent (and the credit).

Fix

• Implement the line-5c fallback: when utilities_included_in_rent is true and the utility amount is unknown (utility_expense is 0), deduct 15% of gross rent before applying the 15% rent-constituting-property-tax rate.
• Add parameter gov.states.me.tax.income.credits.fairness.property_tax.rate.utilities_included_in_rent (0.15) for the utility fraction.
• Add a countable-rent test for the unknown-amount case.

Example

Rent $17,139 with utilities included, amount unknown: utility portion = 15% × 17,139 = 2,571; countable rent = 15% × (17,139 − 2,571) = 2,185, matching the Maine worksheet (previously PE used the full 15% × 17,139 = 2,571). The known-amount and utilities-not-included cases are unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a6acf1f) to head (cafacda).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            5         1    -4     
  Lines           83        18   -65     
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- Hits            83        18   -65     

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

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