Implement Medicaid medically needy/spenddown pathway for aged, blind, and disabled people

## Problem

SSI asset-limit reform analysis showed that many older/disabled people who newly receive SSI are not Medicaid-eligible through non-SSI pathways in the current model. That is partly real because non-MAGI aged/blind/disabled Medicaid can use resource tests, but the model is missing or not integrating key alternative pathways, especially medically needy/spenddown.

Related closed issues: #6109 and #6110 had 2015 medically needy income/resource limit data; #6010 is now reopened as the umbrella.

## Why this matters

In the local 2026 enhanced CPS no-SSI-asset-limit run:

- +618.5k people became Medicaid eligible through the SSI-recipient pathway.
- +584.1k became Medicaid enrolled.
- Medicaid cost rose +$20.1B/year.
- Among the new SSI-linked Medicaid group, nearly all were income-eligible for optional aged/disabled Medicaid but 0 were asset-eligible.

Some of these people may instead qualify under medically needy/spenddown rules, especially people with high medical expenses. Current results therefore likely overstate the cliff between not receiving SSI and receiving SSI for Medicaid access.

## Scope

Model a first-pass medically needy eligibility category for aged/blind/disabled people, including:

- State availability and medically needy income levels.
- State resource limits where applicable.
- Spenddown / incurred medical expense logic if feasible from available variables.
- Integration into `medicaid_category` / `is_medicaid_eligible` without double-counting existing SSI, optional senior/disabled, and MAGI categories.
- Tests for states with known medically needy pathways and states without one.

## Sources

- KFF non-MAGI Medicaid eligibility surveys.
- Existing issue data in #6109 and #6110.
- Federal framework: 42 CFR 435.301 and related medically needy rules.

## Acceptance criteria

- People meeting state medically needy rules can become Medicaid-eligible even without SSI receipt.
- Existing SSI-recipient and optional senior/disabled behavior remains stable in tests.
- Add documentation explaining which medically needy variants are modeled and which remain omitted.