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.
• Keeps the rule household-calculator-only for now; this PR does not wire the reduction into baseline property-tax-credit aggregates while assessed-property-value data readiness is unresolved.

Closes #8196.

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/ky/tax/property/homestead_exemption -c policyengine_us
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/ruff check policyengine_us/variables/gov/states/ky/tax/property/homestead_exemption policyengine_us/variables/gov/states/ky/tax/income/credits/ky_refundable_credits.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (eee0f91) to head (1cc08e6).
⚠️ Report is 128 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #8309       +/-   ##
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+ Coverage   78.72%   100.00%   +21.27%     
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  Files        4723         3     -4720     
  Lines       68774        39    -68735     
  Branches      340         0      -340     
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- Hits        54142        39    -54103     
+ Misses      14554         0    -14554     
+ Partials       78         0       -78     

Flag	Coverage Δ
unittests	100.00% <100.00%> (+21.27%)	⬆️

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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.