v2 bootstrap: generic lambda-param instantiation from call-site type — dissolves the 3-instance fold-consumer gap class (design ruling tidy-stag-172, dep-graph-2026-06-12 §4a). Extend v2 R2 minimal generic instantiation so: (a) fold_list/fold_list_right cons|snoc callbacks bind T from xs: FreeMonoid - #4736