Add Indiana over-65 property tax credit

[daphnehanse11]

Summary

• Adds Indiana's Over 65 Property Tax Credit beginning with taxes first due and payable in 2026, based on current DLGF/county guidance.
• Adds age, surviving-spouse age, income-limit, and credit-amount parameters.
• Adds eligibility and credit variables plus focused tests.

Closes #8195.

Value Review

• Indiana DLGF guidance lists a $150 maximum credit.
• Indiana DLGF guidance limits federal AGI to $60,000 for single filers and $70,000 for joint filers or shared owners.
• Indiana DLGF guidance says the credit applies for 2025 Pay 2026 property taxes and uses federal adjusted gross income from the calendar year preceding the credit year by two years.
• DLGF and county guidance list age 65 as the primary age threshold and age 60 for a qualifying surviving, un-remarried spouse.

Sources:

• Indiana DLGF, Legislation Affecting Deductions and Exemptions: https://www.in.gov/dlgf/files/2025-memos/250612-Cockerill-Memo-Legislation-Affecting-Deductions%2C-Exemptions%2C-and-Credits.pdf#page=2
• Monroe County Auditor, Over 65: https://www.in.gov/counties/monroe/Departments/auditor/over-65/

Modeling Notes

• This PR models the flat $150 Over 65 Credit only. The separate Over 65 Circuit Breaker Credit remains follow-up work because it depends on prior-year property tax liability and tax-growth limits.
• The model uses adjusted_gross_income from the prior-prior year as the available federal AGI proxy.
• The model uses positive real_estate_taxes as the available homeowner/property-tax proxy. It does not verify the one-year ownership/application-filing conditions or reduce the credit for partial ownership shares when not all joint tenants or tenants in common are age 65 or older.
• Surviving-spouse eligibility uses the explicit is_surviving_spouse marker when available, while retaining federal surviving-spouse filing status in the credit year as a fallback.

Tests

• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/ruff check policyengine_us/variables/gov/states/in/tax/property/over_65_credit policyengine_us/tests/policy/baseline/gov/states/in/tax/property/over_65_credit
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/states/in/tax/property/over_65_credit -c policyengine_us
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/python -m pytest policyengine_us/tests/test_parameter_files.py policyengine_us/tests/test_system_import.py -q

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (bf8b00a) to head (f43afe6).
⚠️ Report is 70 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #8308       +/-   ##
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+ Coverage   78.70%   100.00%   +21.29%     
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  Files        4750         2     -4748     
  Lines       69270        36    -69234     
  Branches      341         0      -341     
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- Hits        54517        36    -54481     
+ Misses      14675         0    -14675     
+ Partials       78         0       -78     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+21.29%)	⬆️

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