Pass is_tax through to detailed_budget so consumers can sign each program #971

Problem

build_detailed_budget copies baseline_total, reform_total and change from policyengine.py's program statistics but drops is_tax. Consumers have to decide which programs are benefits from their own lists; the v3 app's "Budgetary impact by programme" chart uses a fixed BENEFIT_PROGRAMS set.

PolicyEngine/policyengine.py#724 adds a UK row for every tax and benefit in the budget totals that a reform changes (for example scottish_child_payment and pip). Those aren't in the app's list, so once sim-api deploys a .py release with that change the chart would draw them as taxes, with the wrong sign: on report 4738 (2027 UC freeze), Scottish Child Payment would show −£25m instead of +£25m.

Proposal

Add is_tax: bool | None to DetailedBudgetProgramOutput (null in outputs built before it) and copy it from each program-statistics row, so consumers sign rows without a list of their own. This must ship before policyengine.py#724 reaches sim-api.