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, and include the relief in state property tax credits from 2025.

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

• uv run python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/gov/states/tn/tax/property/elderly_property_tax_relief --batches 1
• .venv/bin/python -m pytest policyengine_us/tests/test_parameter_files.py policyengine_us/tests/test_system_import.py policyengine_us/tests/code_health/parameters.py policyengine_us/tests/code_health/variable_names.py
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/ruff format --check .
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/ruff check .

Closes #8203

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8cb60e7) to head (42977aa).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8307   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            3         3           
  Lines           63        34   -29     
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- Hits            63        34   -29     

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

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