Model working-disabled Medicaid buy-in premiums with state-specific schedules

Follow-up to #8493 and narrower successor to the working-disabled buy-in part of #8102. Eligibility and premium modeling should be split enough that #8493 can land a shared eligibility category without pretending all buy-in premiums have the same shape.

## Why this needs a separate issue

Working-disabled Medicaid buy-in premiums are not just a yes/no or one-rate parameter. Across the states that need state-specific attention, premium rules vary by:

- flat FPL brackets;
- percent of countable earnings;
- percent of countable income;
- percent of gross or adjusted gross income;
- separate earned and unearned income components;
- caps and maximums;
- premium assistance offsets;
- whether premium payment is a condition of eligibility;
- whether retroactive coverage requires retroactive premiums.

Trying to force all of that into the first shared eligibility PR would make #8493 harder to review and would risk silently modeling premiums incorrectly. The first eligibility implementation should expose a clean hook for `working_disabled_buy_in_medicaid_premium` or a broader `medicaid_premium`, then premium schedules can be filled in with focused tests.

## Examples that justify a separate premium framework

### Mississippi Working Disabled

- No premium below 150% FPL.
- Above 150% FPL, monthly premium equals 5% of countable earnings.
- The July 2025 manual says premium payment is a condition of continuing Working Disabled eligibility.
- Premiums for retroactive months must be paid for retroactive coverage.

### Rhode Island Ticket to Work and Sherlock

- Ticket to Work has no income or asset limit, with flat premium brackets from no premium below 150% FPL to $370 at 900%+ FPL.
- Sherlock has a similar lower premium schedule but is bounded by its 250% FPL income limit.
- EOHHS says TTW participants with higher earnings may pay more than the Sherlock Plan's former $92 maximum.

### New Hampshire MEAD

- Monthly premium payment is a condition of MEAD eligibility.
- No premium below 150% FPL.
- Premiums are capped at 7.5% of net income for people at or below 450% FPL.
- Premium schedule tiers use 150%, 200%, 250%, 300%, 350%, and 400% FPL thresholds.
- If annual adjusted gross income exceeds $75,000, the full premium is 7.5% of AGI.
- Nonpayment for 2 consecutive months without good cause terminates MEAD.

### Massachusetts CommonHealth

- Premiums use MassHealth premium billing family group rules.
- CommonHealth young adults/adults above 150% FPL use a full premium formula that keeps increasing above 1000% FPL.
- A supplemental premium formula applies to members with other health insurance when MassHealth does not contribute.
- Premium assistance can offset the CommonHealth premium.
- Employer-sponsored insurance enrollment can be required when MassHealth determines premium assistance applies.

### Illinois HBWD

- Premiums are mandatory.
- Premium amount is based on combined gross unearned income and countable earned income.
- The existing PolicyEngine implementation uses separate earned and unearned component parameters rather than a single flat rate.
- The state table is effectively a two-dimensional premium schedule, with state-specific behavior that should be preserved.

## Implementation notes

- Add or extend a federal aggregator such as `medicaid_premium` or `working_disabled_buy_in_medicaid_premium`.
- Keep eligibility variables separate from premium amount variables, but document states where premium payment is a condition of eligibility.
- Support at least these premium shapes:
- fixed FPL bracket table;
- percent of countable earnings;
- percent of countable or net income with FPL cap;
- earned-plus-unearned component table;
- full vs supplemental premium formula;
- maximum premium cap.
- Allow state-specific premium variables where the shared parameter shape is not enough.
- Avoid subtracting premiums from Medicaid benefit value until the broader medical out-of-pocket architecture says how to handle it.

## Acceptance criteria

- #8493 can expose a premium hook without blocking the shared eligibility category on every state's premium schedule.
- At least one representative premium test exists for:
- Mississippi percent-of-countable-earnings premium;
- Rhode Island flat FPL bracket premium;
- New Hampshire tiered 7.5% premium;
- Illinois earned/unearned component premium, preserving existing tests;
- Massachusetts CommonHealth full premium formula, if Massachusetts is in scope.
- Documentation lists which states have implemented premium formulas and which states are eligibility-only for now.
- Premium treatment is linked to, but not confused with, Medicaid legal eligibility category and cost/risk grouping.

## Sources

- Mississippi Division of Medicaid, Working Disabled: https://medicaid.ms.gov/medicaid-coverage/who-qualifies-for-coverage/working-disabled/
- Rhode Island EOHHS, Medicaid Programs For Working People With Disabilities: https://eohhs.ri.gov/Consumer/TheSherlockPlan.aspx
- N.H. Admin. Code He-C 5003.01, Premium Schedule for MEAD: https://www.law.cornell.edu/regulations/new-hampshire/N-H-Admin-Code-SS-He-C-5003.01
- 130 CMR 506.011, MassHealth Premiums and CMSP Premiums: https://www.law.cornell.edu/regulations/massachusetts/130-CMR-506-011
- Illinois HFS 3792, HBWD Program: https://hfs.illinois.gov/medicalprograms/hbwd/hfs3792hbwd.html

Related: #8493, #8102, #8497.