Port closed parameterless Boolean Func completion callbacks

Opt-in Task scheduling port of a closed explicit Func<bool> local with one ignored BeginInvoke(inline synchronous callback,null) and one EndInvoke initializing the callback's first local, directly or first in a try. Retain the initializer, callback body, receiver capture/lifetime and exact worker exception objects through the shared typed Task/result-envelope helpers. Leave other references, state/result use, named/null-conditional calls, other signatures/shapes, jumps/await/yield, discarded comments, partial/generic owners and snapshots unchanged. Adopt Task scheduling, execution-context/child-task policy, added allocations/frames and unobserved escaping-callback faults. No general APM/remoting/transport/shutdown equivalence; separate from version upgrades.