MatchScaffolding

The scrutinee-repr-driven half of a sum match. A MatchScaffolding captures the Case-builtin shape and any pre-bound state derived from the scrutinee (e.g. an unpacked field-list var, a tag binding) — but it does NOT decide the result repr. assembleMatch calls assemble once it has the aligned branches and the chosen target repr.

One scaffolding implementation per scrutinee repr (Term.Case for UplcConstr, CaseListLoweredValue for SumBuiltinList, …). The trait stays small so future per-repr emitters can implement it without pulling in the assembly machinery.