Add Mississippi age or disability homestead exemption

[daphnehanse11]

Summary

• Adds Mississippi's Tier 2 homestead exemption for age-65-or-disabled homeowners.
• Models eligibility, exempt assessed value, and estimated property-tax reduction for the first $7,500 of assessed value.
• Adds focused eligibility, exemption, and reduction tests.
• 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 #8198.

Value Review

• Mississippi Department of Revenue says the additional age-65-or-disabled exemption applies to the first $7,500 of assessed value.
• HB 1255 in the 2025 Mississippi legislative session proposed increasing this amount to $12,500, but the bill died in committee, so the current modeled value remains $7,500.

Source:

• Mississippi Department of Revenue, Homestead Exemption: https://www.dor.ms.gov/county-services/homestead-exemption

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/ms/tax/property/age_disability_homestead_exemption -c policyengine_us
• /Users/daphnehansell/Documents/GitHub/policyengine-us/.venv/bin/ruff check policyengine_us/variables/gov/states/ms/tax/property/age_disability_homestead_exemption

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (eee0f91) to head (1376618).
⚠️ Report is 128 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main     #8313       +/-   ##
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+ Coverage   78.72%   100.00%   +21.27%     
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  Files        4723         3     -4720     
  Lines       68774        38    -68736     
  Branches      340         0      -340     
============================================
- Hits        54142        38    -54104     
+ Misses      14554         0    -14554     
+ Partials       78         0       -78     

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

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