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, plus wire the relief into state property tax credits from 2025.

Tests

• .venv/bin/python policyengine_us/tests/test_batched.py policyengine_us/tests/policy/baseline/gov/states/al/tax/property/senior_homestead_exemption --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 #8192

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8cb60e7) to head (fb1d6f6).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8304   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         2    -1     
  Lines           63        23   -40     
=========================================
- Hits            63        23   -40     

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

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