Add Alabama senior homestead relief

[daphnehanse11]

Summary

• Add Alabama's senior homestead property tax exemption for the mandatory age-65 state-levy component.
• Parameterize the age threshold and 6.5-mill state property tax rate.
• 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.

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/al/tax/property/senior_homestead_exemption -c policyengine_us
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/ruff check policyengine_us/variables/gov/states/al/tax/property/senior_homestead_exemption policyengine_us/variables/gov/states/al/tax/income/al_refundable_credits.py

Closes #8192

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (eee0f91) to head (fa1669f).
⚠️ Report is 128 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #8304       +/-   ##
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+ Coverage   78.72%   100.00%   +21.27%     
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  Files        4723         2     -4721     
  Lines       68774        23    -68751     
  Branches      340         0      -340     
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- Hits        54142        23    -54119     
+ 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.