Add COFA immigration status for OBBBA SNAP, Medicaid, and ACA eligibility

## Summary

Public Law 119-21 keeps Compacts of Free Association (COFA) residents eligible for the narrowed OBBBA immigration eligibility rules in SNAP, Medicaid, and ACA premium tax credits, but PolicyEngine-US does not currently have a COFA immigration status enum value/input.

Because `ImmigrationStatus` lacks a COFA value, current-law parameters can only list:

- `CITIZEN`
- `LEGAL_PERMANENT_RESIDENT`
- `CUBAN_HAITIAN_ENTRANT`

This means COFA residents cannot be represented as eligible under OBBBA in SNAP/Medicaid/PTC modeling.

## Relevant code/parameters

- `policyengine_us/variables/household/demographic/person/immigration_status.py`
- `policyengine_us/parameters/gov/usda/snap/eligibility/eligible_immigration_statuses.yaml`
- `policyengine_us/parameters/gov/hhs/medicaid/eligibility/eligible_immigration_statuses.yaml`
- `policyengine_us/parameters/gov/aca/ineligible_immigration_statuses.yaml`

SNAP already has a comment noting COFA, but no enum value to include:

```yaml
2025-07-01:
- CITIZEN
- LEGAL_PERMANENT_RESIDENT
- CUBAN_HAITIAN_ENTRANT
# Compacts of Free Association (COFA) citizens
```

## Bill text

Public Law 119-21 includes COFA eligibility in:

- Section 10108, Alien SNAP eligibility: individuals lawfully residing in the United States under a Compact of Free Association
- Section 71109, Alien Medicaid eligibility: same COFA category
- Section 71301, Premium Tax Credit eligible aliens: same COFA category

Official text: https://www.govinfo.gov/content/pkg/PLAW-119publ21/html/PLAW-119publ21.htm

## Suggested fix

Add a COFA value to `ImmigrationStatus`, for example:

```python
COFA = "Compacts of Free Association resident"
```

Then include it in the OBBBA-era eligible-status lists for SNAP and Medicaid, and ensure ACA PTC logic treats COFA lawfully present aliens as eligible rather than ineligible.

Add YAML tests covering COFA individuals for SNAP, Medicaid, and ACA PTC in the effective periods.

Start with policyengine_us/variables/household/demographic/person/immigration_status.py, then inspect the SNAP, Medicaid, and ACA eligibility YAML files listed in the issue and their existing test patterns. Add the COFA status and effective-period rules, then run the relevant YAML tests; done means COFA individuals are represented correctly for SNAP, Medicaid, and ACA PTC eligibility.

Written by the indexing model from the issue text.