Port closed three-argument Action completion callbacks

Opt-in Task scheduling port of an explicit closed Action<T1,T2,T3> local used only by one ignored BeginInvoke and one expression-bodied ar=>action.EndInvoke(ar) callback, with literal null state and no other result/delegate references. Capture receiver/arguments/callback once in order; preserve worker exception identity with ExceptionDispatchInfo and End receiver lifetime. Supported by-value primitives/enums/nongeneric references only. Leave other signatures/shapes, state/result uses, escapes/reassignments, named/null-conditional calls, comments, partial/generic owners and snapshots unchanged. Adopt Task/context/child-task scheduling, new allocations/frames and unobserved escaping-callback faults; not general APM/remoting/transport/shutdown emulation. Separate from version upgrades.