Move families onto Universal Credit when a legacy benefit they receive closes #2118

Problem

Tax credits closed on 6 April 2025, and Income Support and income-based JSA on 1 April 2026 (gov.dwp.tax_credits.active, gov.dwp.income_support.active, gov.dwp.JSA.income.active). DWP moved each working-age family off its legacy benefits with a migration notice covering all of them. The family's awards ended when it claimed Universal Credit (SI 2014/1230 reg 8), or at the notice's deadline if it did not claim (reg 46).

The model has no such step:

• Universal Credit is paid only through would_claim_uc. Datasets draw it once, without regard to legacy receipt, and copy it into every year.
• A legacy family whose draw is false loses its tax credits in 2025-26 and gets no Universal Credit.
• Its other legacy awards carry on: the Housing Benefit continuing award, Income Support, income-related ESA and income-based JSA.

Proposal

• Add a dataset input would_claim_uc_at_legacy_closure (default true, like would_claim_uc). Move working-age legacy-benefit reporters onto UC at Move to UC claim rates (stranded after HB, tax credit, IS and JSA closure) policyengine-uk-data#492 draws it at DWP's Move to Universal Credit claim rates.
• Add legacy_benefits_closed: a working-age family reports a legacy benefit whose active parameter is false. Families on the pension-age route are excluded (reg 8(2B)).
• Add claims_universal_credit = would_claim_uc | (legacy_benefits_closed & the new input), and use it wherever the model reads would_claim_uc as "this family claims Universal Credit".
• Once a family's legacy benefits have closed, end its Housing Benefit continuing award, Income Support, income-related ESA and income-based JSA, whether or not it claims.

Years before any closure stay as they are.

Housing Benefit (#1910) and income-related ESA (#1914) join legacy_benefits_closed when the model ends their working-age awards.