Tasks and events no longer interrupt synchronous Objo code

Tasks, timers, host events, and desktop control events can currently interrupt ordinary Objo code between bytecode instructions. This contradicts the cooperative-concurrency contract that synchronous code runs uninterrupted until it returns to the scheduler or reaches a pending Await. The VM should defer queued work to a scheduler boundary so multi-statement state changes cannot expose intermediate state and Task.Yield genuinely yields.