Add Nevada TANF Employment Retention Payment (A-660.12 transitional benefit)

Split out from #8771, whose rounding and minimum-issuance parts are fixed by #8925.

Nevada's Employment Retention Payment (E&P Manual A-660.12 / A-800) pays $50/month for up to 6 months to households whose TANF case **terminated for excess income** with at least $1 of earned income. It is a post-eligibility transitional benefit:

- The recipient household is, by definition, no longer TANF-eligible — so it cannot be modeled inside `nv_tanf`, which is `defined_for: nv_tanf_eligible`.
- Determining it requires prior-period program state (was the unit on TANF, did the case close for excess income, months since termination) that the static annual snapshot does not carry.

Implementing it would mean a separate `nv_tanf_employment_retention_payment` variable with an explicit modeling assumption for the termination-history inputs (e.g., user-supplied month-of-termination, or an imputation), similar to how other transitional benefits are handled.

Reference: NV DWSS E&P Manual, chapter A-600 (Budgeting), A-660.12, https://dwss.nv.gov/siteassets/dwss.nv.gov/ep-manual-section-a/chapter-a-600-budgeting-ada-8.2025.pdf#page=38

Start by reading the existing nv_tanf variable and the Nevada DWSS E&P Manual sections A-660.12 and A-800. Determine how termination history can be represented in the static annual snapshot and document an explicit assumption for those inputs. Done means a separate nv_tanf_employment_retention_payment variable models $50 per month for up to six months under the stated termination conditions.

Written by the indexing model from the issue text.