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

## 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 #9330 covers SSI, SNAP, Medicaid and HUD; Medicare is not on its list.

Start by locating the is_medicare_eligible formula and the declarations for its existing age, social_security_disability, and months_receiving_social_security_disability inputs. Review the cited Medicare routes and existing eligibility tests, then add coverage for ESRD, ALS, and the SSDI-months documentation; done means the new boolean inputs and documented defaults produce the specified eligibility outcomes.

Written by the indexing model from the issue text.