Model New Mexico Native American premium enhancements (MAP Table 2 tribal schedule + Premium Buy-down)

### Summary
New Mexico's Native American premium enhancements are unmodeled in PolicyEngine. Given NM's ~10%+ AI/AN population, this is a non-trivial statewide gap.

### Details (PY2026 MAP Manual; continue in PY2027)
- **Table 2 (p. 5)** replaces Table 1 for tribal members: 0% contribution from 0–300% FPL; 1–8.5% from 300–400% FPL, computed on the **lowest-cost plan per issuer** (not the SLCSP benchmark).
- **Premium Buy-down (pp. 6–7)** guarantees $0 premiums for Zero Cost Sharing Variant qualifiers.

### Suggested approach
- Minimum (done in #9232): a Not-Modeled entry in the PR description and variable docs.
- This issue: implement the Table 2 tribal schedule (the 1–8.5% scale fits the existing interpolation machinery if a tribal-membership variable exists) and the Premium Buy-down $0 guarantee.

_Filed per review item A7 (reviewer @hua7450), surfaced reviewing NM Premium Assistance PR #9232._

Start by reviewing PR #9232 and the PY2026 MAP Manual details cited in this issue, especially Table 2 on page 5 and the Premium Buy-down on pages 6–7. Trace the existing interpolation machinery and determine whether a tribal-membership variable already exists; done means the tribal schedule and the $0 guarantee are represented and covered by the project's relevant checks.

Written by the indexing model from the issue text.