AR Medicaid work requirement misses SSI-criteria disability in medically-frail exemption

Part of #9330 (disconnected disability inputs audit). State analogue of #9329.

## Summary

Arkansas's Medicaid work requirement has the same defect as the federal variable in #9329: the disability exemption reads only the generic inputs, missing people flagged disabled under SSI's criteria — who are medically frail by federal definition.

## Current code

`policyengine_us/variables/gov/states/ar/dhs/medicaid/eligibility/ar_medicaid_work_requirement_eligible.py:34`:

```python
eligible_disabled = is_disabled | is_blind | is_incapable_of_self_care
```

(same trio in the `has_disabled_dependent` carer prong)

## Legal basis

Arkansas Works exempts those identified as medically frail. [42 CFR 440.315(f)](https://www.law.cornell.edu/cfr/text/42/440.315) requires the medically-frail definition to at minimum include "individuals with a disability determination based on Social Security criteria or in States that apply more restrictive criteria than the Supplemental Security Income program, the State plan criteria".

## Impact

A person with `meets_ssi_disability_criteria = True` but `is_disabled = False` is subjected to the 80-hour community-engagement test despite being medically frail by construction.

## Proposed fix

Add `meets_ssi_disability_criteria` to `eligible_disabled` in both the self-exemption and the dependent-carer prong (mirroring whatever fix lands for the federal variable in #9329), plus a regression test.

Start with policyengine_us/variables/gov/states/ar/dhs/medicaid/eligibility/ar_medicaid_work_requirement_eligible.py at line 34 and inspect both disabled eligibility expressions. Compare the federal analogue referenced in #9329, then add a regression test covering meets_ssi_disability_criteria for self-exemption and dependent-carer eligibility; done means medically frail people are not subjected to the 80-hour test.

Written by the indexing model from the issue text.