Add Tennessee elderly property tax relief

[daphnehanse11]

Summary

• Add Tennessee Property Tax Relief for Elderly Homeowners.
• Parameterize age, residential assessment rate, income limits, and qualifying property value cap.
• Include 2025 and 2026 values: income limit 37,530 in 2025 and 38,470 in 2026; property value cap 32,700 for both years.
• Add focused eligibility and amount tests.
• Keep the rule household-calculator-only for now; this PR does not wire the relief into refundable or property-tax-credit aggregates while assessed-property-value data readiness is unresolved.

Modeling notes

• Uses real_estate_taxes to preserve local tax-rate differences.
• Converts the published full-market-value cap to an assessed-value cap using Tennessee's 25% residential assessment rate.
• Uses adjusted gross income as the modeled tax-unit income proxy.

Tests

• PYTHONPATH=. /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/states/tn/tax/property/elderly_property_tax_relief -c policyengine_us
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/ruff check policyengine_us/variables/gov/states/tn/tax/property/elderly_property_tax_relief

Closes #8203

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (61fead1) to head (fed7669).
⚠️ Report is 94 commits behind head on main.

Additional details and impacted files

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- Hits           338        34   -304     

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

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[MaxGhenis]

Converted this to draft for the same data-readiness reason as #8303, but scoped to property tax data. This PR relies on assessed_property_value, which is currently only a PolicyEngine-US input variable and is not present/imputed in policyengine-us-data; rent and real_estate_taxes are data-backed, but assessed value is not. The rule tests are still useful, but baseline wiring into refundable/property-tax-credit aggregates should wait until we either add an assessed-value data/imputation path or revise the formula to rely only on data-backed inputs.