enum MainQueue

Work handed to the main thread from another one.

The reason this has to exist, rather than a worker simply writing @State when it finishes: StateStorage is @Observable, and withObservationTracking's onChange fires synchronously on whichever thread performed the write. A background write would therefore run ViewInvalidation.markBodyDirty on that thread — and ViewInvalidation, FrameScheduler and FrameTasks are all single-threaded statics the run loop reads every iteration. The race is real, not theoretical.

So the division is: a worker computes pure Sendable values and posts the application of them here. async is the only member safe to call from another thread, and everything it enqueues runs on the main thread.

AgentServer already had the shape of this — watcher thread, lock, wake the loop, do the work on the main thread — but kept it private and specific to its socket.