Audit SSI-recipient Medicaid state handling: 1634, SSI-criteria, and 209(b) states

## Problem

The current SSI-recipient Medicaid category is a single `ssi_recipient.is_covered[state]` boolean. That is too coarse for SSI reform analysis.

The current implementation also appears inconsistent with SSA POMS: Oklahoma is marked false in `gov.hhs.medicaid.eligibility.categories.ssi_recipient.is_covered`, but SSA POMS lists Oklahoma as an SSI-criteria state rather than a 209(b) state. SSI-criteria states require state Medicaid determinations but use SSI criteria; 209(b) states can use more restrictive rules.

## Why this matters

In the local 2026 no-SSI-asset-limit reform:

- +1.399M people newly receive SSI in the current model.
- +618.5k become Medicaid eligible through the SSI-recipient pathway.
- +584.1k become Medicaid enrolled.
- Medicaid cost rises +$20.1B/year.

The size and state distribution of this Medicaid effect depend heavily on how we treat 1634, SSI-criteria, and 209(b) states.

## Scope

- Replace or supplement the boolean with explicit state classification: 1634, SSI-criteria, 209(b).
- Audit Oklahoma and all other states against SSA POMS SI 01715.010 / SI 01715.020.
- For SSI-criteria states, decide whether SSI receipt should imply Medicaid eligibility in the annual model or whether a separate application/take-up gate is needed.
- For 209(b) states, model the relevant aged/blind/disabled Medicaid criteria instead of blanket excluding all SSI recipients.
- Preserve current behavior only where legally justified.

## Sources

- SSA POMS SI 01715.010: Medicaid and SSI recipient state classifications.
- SSA POMS SI 01715.020: 209(b) and SSI-criteria states.
- 42 U.S.C. 1396a(f) / 42 CFR 435.121 for more restrictive 209(b) rules.

## Acceptance criteria

- State classification is represented explicitly in parameters.
- Tests cover at least one 1634 state, one SSI-criteria state, Oklahoma, and one 209(b) state.
- SSI asset-limit reform Medicaid effects are not driven by an undocumented boolean shortcut.

Start by reading SSA POMS SI 01715.010 and SI 01715.020, then locate the parameter implementing gov.hhs.medicaid.eligibility.categories.ssi_recipient.is_covered and its existing tests. Compare every state classification, including Oklahoma, with the cited sources and determine the SSI-criteria application gate and 209(b) criteria. Done means explicit 1634, SSI-criteria, and 209(b) parameters, representative tests, and no undocumented boolean shortcut driving reform results.

Written by the indexing model from the issue text.