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 notes the credit income limits are not annually adjusted by Social Security COLA.
• 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 as the available federal AGI proxy.
• The model uses positive real_estate_taxes as the available homeowner/property-tax proxy and does not reduce the credit for partial ownership shares when not all joint tenants or tenants in common are age 65 or older.

Tests

• uv run policyengine-core test policyengine_us/tests/policy/baseline/gov/states/in/tax/property/over_65_credit -c policyengine_us
• uv run pytest policyengine_us/tests/test_parameter_files.py policyengine_us/tests/test_system_import.py -q
• uv run --extra dev ruff check policyengine_us/variables/gov/states/in/tax/property/over_65_credit

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8cb60e7) to head (6ef0c7e).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8308   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         2    -1     
  Lines           63        31   -32     
=========================================
- Hits            63        31   -32     

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

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.