Medicare eligibility: add the ESRD and ALS routes; document the SSDI-months input

[MaxGhenis]

Summary

is_medicare_eligible models two routes: age 65 or older, and social_security_disability > 0 with months_receiving_social_security_disability >= 24 (42 USC 426(a)(1) and 426(b)(2)(A)). The statute has two more under-65 routes the formula omits:

• End-stage renal disease (42 USC 426-1): entitlement after a waiting period tied to dialysis or transplant, with no 24-month requirement.
• ALS (42 USC 426(h)): the 24-month waiting period is waived.

Neither route has an input today. Suggested shape: has_esrd and has_als boolean inputs (default false), OR'd into is_medicare_eligible, with the SSDI-months route unchanged.

Two related notes from PolicyBench's audit of its frozen references (PolicyEngine/policybench#164):

• months_receiving_social_security_disability is an integer input that no microdata build carries, so the disability route never fires in simulation; the certified builds record a year of SSDI receipt for 3,190 persons with no duration. Documenting the input's default and its interaction with year-round SSDI receipt (a zero-months default is an incoherent description of a year-round recipient) would help downstream users.
• The disability-wiring audit in Disconnected disability inputs: repo-wide audit of disability gates vs their legal definitions #9330 covers SSI, SNAP, Medicaid and HUD; Medicare is not on its list.