CHIP immigration eligibility does not encode H.R.1 §71109 narrowing (Medicaid side is encoded; CHIP check is only ~undocumented)

H.R.1 §71109 ("Restricting Immigrant Eligibility for Medicaid and CHIP") narrows qualified-immigrant eligibility for **both programs** to lawful permanent residents, certain Cuban/Haitian entrants, and COFA citizens (plus lawfully residing children/pregnant adults in ICHIA states), effective **2026-10-01** (KFF, Health Provisions in the 2025 Federal Budget Reconciliation Law).

Current state of the model (verified at 1.772.0 and at HEAD 715373c90b):

- **Medicaid**: encoded — `parameters/gov/hhs/medicaid/eligibility/eligible_immigration_statuses.yaml` switches to `[CITIZEN, LEGAL_PERMANENT_RESIDENT, CUBAN_HAITIAN_ENTRANT]` at 2026-10-01.
- **ACA PTC**: encoded — `gov/aca/below_fpl_immigration_exception_in_effect` false from 2026 (§71302(b)); `gov/aca/ineligible_immigration_statuses` expands at 2027-01-01.
- **CHIP**: not encoded — `variables/gov/hhs/chip/is_chip_eligible_child.py` checks only `immigration_eligible = ~undocumented`, with no dated parameter.

Suggested fix: route CHIP's immigration check through the same dated eligible-statuses parameter surface as Medicaid (or a CHIP-specific parameter with the §71109 2026-10-01 value), so the narrowing binds for CHIP from October 2026. Same applies to the pregnant-person and FCEP CHIP paths if they carry their own checks.

Found while preparing the IARIW 2026 OBBBA paper (PolicyEngine/obbba-paper); companion data-side ask (the humanitarian categories are absent from the microdata entirely) is PolicyEngine/microcosm#767.

Start with variables/gov/hhs/chip/is_chip_eligible_child.py and compare its immigration check with parameters/gov/hhs/medicaid/eligibility/eligible_immigration_statuses.yaml. Search the pregnant-person and FCEP CHIP paths for separate checks. Done means CHIP immigration eligibility uses a dated status parameter and applies the narrowed statuses from 2026-10-01.

Written by the indexing model from the issue text.