Model Medicaid Buy-In eligibility for working disabled people beyond Illinois

## Problem

PolicyEngine has some work around Medicaid premiums and Illinois HBWD, but SSI reform analysis highlighted a broader gap: working disabled Medicaid Buy-In pathways can provide non-SSI Medicaid eligibility to disabled adults who fail standard SSI/optional ABD resource or income tests.

The current national Medicaid category set does not systematically include Buy-In eligibility. This can overstate Medicaid loss/cliffs for disabled people who do not receive SSI.

## Why this matters

In the local 2026 no-SSI-asset-limit run, PolicyEngine found +618.5k new Medicaid eligible people through SSI receipt and +$20.1B/year Medicaid cost. Some working disabled people may have non-SSI Medicaid access through Buy-In programs instead, especially in states with higher income/resource thresholds.

## Scope

- Inventory state Medicaid Buy-In / working disabled programs and prioritize high-population states.
- Add a federal aggregator/category or state-specific variables for Buy-In eligibility.
- Reuse existing state implementations where present, starting with Illinois HBWD if it is already modeled.
- Decide how Buy-In premiums interact with `medicaid_premium` and `healthcare_benefit_value`.
- Add tests for eligibility and integration into `is_medicaid_eligible` / `medicaid_category`.

## Related

- #6010 umbrella senior/disabled Medicaid completeness.
- #8102 Medicaid premiums and Working Disabled Buy-In discussion.
- #6783 Illinois HBWD implementation notes.

## Acceptance criteria

- At least one non-Illinois Buy-In pathway is modeled end-to-end.
- `medicaid_category` or a parallel categorization can distinguish Buy-In from SSI-recipient and optional ABD pathways.
- Results document whether the pathway provides full Medicaid benefits and whether premiums are netted out.