Add Arkansas age 65 or disabled homeowner property tax relief

## Summary

Add Arkansas age-65 or disabled homeowner property tax relief.

## Why add this

- AARP and Arkansas DFA identify this as statewide senior homeowner relief.
- It appears missing from `policyengine-us`.

## Modeling notes

- This is an assessed-value freeze rather than a direct rebate.
- It belongs in the comparison inventory, but it is not a top-tier first modeling target.
- If implemented, treat it as a freeze-style property tax benefit rather than a simple current-year credit.

## Likely inputs

- `age`
- homeownership / principal residence status
- `assessed_property_value`
- prior/base-year value information if full freeze mechanics are modeled

## Sources

- https://www.dfa.arkansas.gov/office/arkansas-assessment-coordination-division/real-property/property-tax-relief/
- https://my.aarpfoundation.org/property-taxes/taxpayer-states/arkansas/