Investigate USDA/SNAP disabled status modeling and data support

## Context

While making `meets_ssi_disability_criteria` input-only, we noticed that SNAP fixtures should not use SSI disability criteria as a proxy for SNAP elderly/disabled status. SNAP has a separate `is_usda_disabled` concept, and USDA/SNAP disability rules are not the same as SSI disability criteria.

Current PE-US structure is directionally right: `has_usda_elderly_disabled` uses `is_usda_disabled`, and `is_usda_disabled` is driven by `gov.usda.disabled_programs`, currently including `is_ssi_disabled`, `social_security_disability`, permanent disabled veteran, and surviving spouse/child of disabled veteran. But the statutory definition in 7 U.S.C. § 2012(j) appears broader and program/status-based, not a general functional limitation or raw SSI medical-criteria screen.

## Why this matters

SNAP special rules for elderly/disabled households affect the gross income test, asset test, excess shelter deduction cap, medical expense deduction, and work/student rules. We should avoid using `meets_ssi_disability_criteria` or generic `is_disabled` as a shortcut for SNAP disability status unless the law supports that specific path.

## Things to investigate

- Compare PE-US `is_usda_disabled` against the full 7 U.S.C. § 2012(j) elderly/disabled member definition and FNS guidance.
- Identify which statutory qualifying statuses are currently missing from PE-US, e.g. state disability-based assistance, qualifying Medicaid/disability-related medical assistance, government disability retirement, Railroad Retirement disability annuity, and the full VA/survivor categories.
- Decide whether PE-US needs additional input variables or formulas for missing USDA qualifying statuses.
- In policyengine-us-data, check whether CPS/SIPP contain source variables for any missing USDA qualifying statuses.
- If source variables are missing, decide whether they should be imputed separately from SSI disability criteria, or left unmodeled with documentation.
- Ensure SNAP tests/fixtures use `is_usda_disabled` or qualifying program/status inputs directly, not `meets_ssi_disability_criteria`.

## References

- 7 U.S.C. § 2012(j): https://www.law.cornell.edu/uscode/text/7/2012
- FNS SNAP special rules for elderly/disabled households: https://www.fns.usda.gov/snap/eligibility/elderly-disabled-special-rules