Model Massachusetts CommonHealth separately from the shared working-disabled buy-in schema

Follow-up to #8493 and narrower successor to closed #8465. Massachusetts CommonHealth should be modeled separately from the first shared working-disabled buy-in schema because it is a state coverage type for disabled children, disabled adults, disabled working adults, and disabled young adults, not only a simple disabled-worker buy-in row.

## Why this is separate from the shared schema

MassHealth CommonHealth has several features that do not fit neatly into a first-pass shared working-disabled buy-in parameter table:

- It covers multiple disabled groups, not just working disabled adults.
- Disabled working adults have a specific work-hours rule: at least 40 hours per month, or at least 240 hours in the six months before application or review if working fewer than 40 hours/month.
- The disability definition for working adults ignores substantial gainful activity, but the person must be permanently and totally disabled.
- Disabled adults who are not working can qualify through a one-time deductible route, or with HIV-positive status and income up to 200% FPL.
- There are separate rules for disabled working young adults, disabled young adults, disabled 18-year-olds, and disabled children.
- Applicants and members must use available health insurance, including employer-sponsored coverage and Medicare, when MassHealth requires it and premium assistance applies.
- CommonHealth premiums use MassHealth premium billing family group rules, full vs supplemental premium formulas, and premium assistance offsets.
- CommonHealth members can have extended coverage for up to three calendar months after terminating employment if premiums are paid timely.

This should probably feed into the shared `WORKING_DISABLED_BUY_IN` category only for the working-disabled subpopulation, while preserving a Massachusetts-specific CommonHealth implementation for the broader coverage type.

## Source details

130 CMR 505.004 says MassHealth CommonHealth is available to disabled children, disabled adults, and disabled working adults.

Disabled working adults must:

- be age 21 through 64;
- be employed at least 40 hours per month, or if employed less than 40 hours/month, have been employed at least 240 hours in the six-month period immediately before application or eligibility review;
- be permanently and totally disabled, except for engagement in substantial gainful activity;
- be a U.S. citizen or qualified noncitizen;
- be ineligible for MassHealth Standard;
- comply with health-insurance-use requirements.

Disabled adults who are not in the working-adult group must:

- be age 21 through 64;
- be permanently and totally disabled;
- be ineligible for MassHealth Standard;
- be a citizen or qualified noncitizen;
- either meet a one-time deductible or have Disabled Adult household MAGI at or below 200% FPL and verify HIV-positive status.

Other CommonHealth groups include disabled working young adults, disabled young adults, disabled 18-year-olds, and disabled children under 18.

Disability can be established by:

- certification of legal blindness by the Massachusetts Commission for the Blind;
- SSA disability determination; or
- Disability Evaluation Services determination.

Premium and insurance rules:

- CommonHealth members may be assessed premiums under 130 CMR 506.011(B)(2).
- The MassHealth premium rule applies to Standard, CommonHealth, and Family Assistance members with income above 150% FPL.
- Premiums are calculated using premium billing family group rules.
- For CommonHealth young adults/adults above 150% FPL, the full premium formula starts at $15 above 150% FPL and increases by FPL brackets, continuing above 1000% FPL.
- A supplemental premium formula applies when members have other health insurance to which MassHealth does not contribute.
- If MassHealth determines a member has employer-sponsored insurance available and it meets premium-assistance criteria, the member generally must enroll; failure to enroll can cause loss or denial of eligibility.
- The regulation provides hardship waiver and delinquent-payment rules.

## Implementation notes

- Do not force all of CommonHealth into `working_disabled_buy_in` if #8493 only models working-disabled buy-in eligibility.
- Create Massachusetts-specific CommonHealth variables, then expose a working-disabled CommonHealth sub-result to the shared `WORKING_DISABLED_BUY_IN` category where appropriate.
- Treat disabled working adult eligibility separately from disabled adult deductible eligibility.
- Model or document the employment test:
- 40 hours/month; or
- 240 hours in the preceding six months.
- Decide whether the first implementation approximates the six-month work-history rule using current annual/monthly employment inputs.
- Model CommonHealth premiums in a premium framework rather than as a flat KFF value.
- Add source comments around premium assistance/employer coverage requirements, because these may be hard to observe in CPS.

## Acceptance criteria

- A Massachusetts disabled working adult age 21-64 who is ineligible for MassHealth Standard and satisfies the work test qualifies through CommonHealth.
- A disabled working adult does not fail merely because their work would otherwise indicate substantial gainful activity.
- A disabled adult who is not working is not incorrectly classified as a working-disabled buy-in case solely because CommonHealth exists.
- The working-disabled CommonHealth path maps to the shared `WORKING_DISABLED_BUY_IN` category from #8493, while leaving room for broader CommonHealth coverage modeling.
- Tests cover:
- age boundary for adult working eligibility;
- 40-hour/month work test;
- premium above 150% FPL if premiums are in scope;
- a non-working disabled adult case to prevent overclassification.

## Sources

- MassHealth CommonHealth, Mass.gov: https://www.mass.gov/info-details/masshealth-commonhealth
- 130 CMR 505.004, MassHealth CommonHealth: https://www.law.cornell.edu/regulations/massachusetts/130-CMR-505-004
- 130 CMR 506.011, MassHealth Premiums and CMSP Premiums: https://www.law.cornell.edu/regulations/massachusetts/130-CMR-506-011
- MassHealth Premium Information for Members: https://www.mass.gov/info-details/masshealth-premium-information-for-members

Related: #8493, #8465, #8102, #8497.