Add Nevada optional state supplementation for 2026

[hua7450]

What and why

Nevada's optional SSI supplement was missing from PolicyEngine-US. Add nv_oss with verified 2026 state-only rates for independent living, another person's household, and state-certified domiciliary care. Reuse the federal SSI eligibility, income exclusions, resources, and couple calculations; add Nevada's care-setting distinctions and payment tables.

The calculation includes mixed-category couples, spouse-to-spouse deeming, cent rounding, and the $1 minimum positive payment. It permits state-only payments after federal SSI is exhausted: for example, an aged individual with $1,030 monthly Social Security has $1,010 countable income, receives no federal SSI, and receives $20.40 Nevada OSS. Add the supplement to household/SPM benefit totals and SNAP unearned income.

References

• SSA's January 2026 payment table, Nevada section I: individual and couple rates, including mixed categories.
• Nevada regional POMS: living arrangements, facility exclusions, and the separate mandatory supplement.
• 20 CFR 416.2025: income reduces the federal payment before the state supplement.
• POMS SI 01320.430: full couple rate for spouse deeming, capped at the claimant's own-income payment.
• POMS SI 02005.030 G: joint couple computation and equal payment division.
• POMS SI 02001.005 and SI 02001.010: minimum payments and rounding each member's payment.

Coverage limits

The registry marks Nevada as partial, verified for 2026. This implements optional supplementation; it does not implement individualized minimum-income protection for recipients converted in December 1973. It inherits the existing federal SSI eligibility and income framework. Earlier years use parameter backfilling and are unverified historical estimates; no historical rate coverage is claimed.

Test plan

• 69 Nevada YAML cases: every published individual/couple rate, age and resource boundaries, income phaseout, state-only payments, minimums and rounding, spouse deeming, facility exclusions, multiple households, and annual benefit/SNAP integration. Expected rates come from SSA's table; income cases show independent arithmetic.
• 169 existing federal SSI and SNAP state-supplement tests passed.
• 650 parameter/registry tests passed; 10 skipped.
• Existing default-dataset microsimulation test passed for 2024–2026 (1,000-household subsample).
• make format and git diff --check.
• Changelog fragment in changelog.d/.

axiom: needed