Add Kentucky homestead property tax exemption

[daphnehanse11]

Summary

• Adds Kentucky's Homestead Exemption for senior or disabled homeowners using the 2025-2026 exemption amount.
• Models eligibility, exempt assessed value, and a property-tax reduction using assessed property value and real-estate taxes.
• Adds focused tests for eligibility, capped exempt value, and tax reduction.

Closes #8196.

Tests

• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/gov/states/ky/tax/property/homestead_exemption --batches 1
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/ruff check policyengine_us/variables/gov/states/ky/tax/property/homestead_exemption
• /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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8cb60e7) to head (30b1c17).
⚠️ Report is 47 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8309   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            3         4    +1     
  Lines           63        48   -15     
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- Hits            63        48   -15     

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.