Update Michigan SSI-related Medicaid asset limits and ABD pathways

## Problem

In the SSI asset-limit reform audit, a large Michigan cluster becomes Medicaid enrolled only after SSI receipt turns on, even though many look plausibly eligible for Michigan Medicaid at baseline through non-SSI pathways.

Using the rebuilt 2024 enhanced CPS and `policyengine-us==1.705.16`, the $10k/$20k SSI asset-limit reform shows about **121k weighted Michigan people** newly Medicaid-enrolled because `is_ssi_recipient_for_medicaid` turns on. Mean SSI-countable resources in that bucket are about **$6.1k**, and about **64k** are Medicare-enrolled at baseline.

## Why this looks like a model gap

Michigan appears to have updated several SSI-related/ABD Medicaid asset limits effective February 1, 2025. MDHHS BEM 400 and the 2024 policy notice indicate 2026 asset limits aligned with MSP levels for many categories: **$9,950 individual / $14,910 couple**. That is much higher than the old SSI $2,000/$3,000 resource limit.

Relevant pathways to verify and encode/update:

- **AD-Care**: full Medicaid for aged or disabled people with net income up to 100% FPL. 2026 table gives monthly limits around **$1,350 individual / $1,824 couple**, with BEM 400 assets.
- **Group 2 Aged, Blind, and Disabled**: aged/blind/disabled Medicaid with monthly deductible/spenddown when income exceeds the need standard.
- **MSP/QMB-SLMB-QI**: likely explains many Medicare-enrolled baseline-ineligible cases. QMB is broader than Part B premium-only assistance because it also pays deductibles and coinsurance.
- **Healthy Michigan Plan**: may explain some under-65 non-Medicare low-income people, but it should exclude people enrolled in or eligible for Medicare.
- **Freedom to Work**: may cover some disabled workers age 16-64, with income up to 250% FPL and assets tied to MSP limits initially.

## Sources

- MDHHS BEM 150, SSI recipients: https://dhhs.michigan.gov/olmweb/ex/BP/Public/BEM/150.pdf
- MDHHS BEM 400, assets: https://dhhs.michigan.gov/olmweb/ex/BP/Public/BEM/400.pdf
- MDHHS 2024 policy notice raising asset limits: https://www.michigan.gov/mdhhs/-/media/Project/Websites/mdhhs/Assistance-Programs/Medicaid-BPHASA/Public-Comment/2024/2449-Eligibility-P.pdf
- MDHHS BEM 163, AD-Care: https://dhhs.michigan.gov/olmweb/ex/BP/Public/BEM/163.pdf
- MDHHS RFT 242, 2026 income/asset tables: https://dhhs.michigan.gov/olmweb/exf/RF/Public/RFT/242.pdf
- MDHHS BEM 166, Group 2 ABD: https://dhhs.michigan.gov/olmweb/ex/BP/Public/BEM/166.pdf
- MDHHS BEM 545, spenddown: https://dhhs.michigan.gov/OLMWEB/EX/BP/Public/BEM/545.pdf
- MDHHS BEM 165, MSP: https://dhhs.michigan.gov/olmweb/ex/bp/public/bem/165.pdf
- Healthy Michigan eligibility: https://www.michigan.gov/en/healthymiplan/who
- MDHHS BEM 174, Freedom to Work: https://dhhs.michigan.gov/olmweb/ex/BP/Public/BEM/174.pdf

## Acceptance criteria

- Verify Michigan's current-law 2025-2026 ABD/SSI-related Medicaid asset limits and update `policyengine-us` parameters/formulas as needed.
- Add or update tests for AD-Care/full ABD Medicaid eligibility around resources above $2,000 but below the 2026 Michigan limit.
- Verify Medicare-enrolled cases are not incorrectly treated as ineligible when they should qualify for QMB/full-benefit dual coverage, and separately represent premium-only MSP where appropriate.
- Verify Healthy Michigan excludes Medicare-eligible/enrolled people but still covers eligible low-income non-Medicare adults without an asset test.