main_loop.fast_events does not manifest as K_EVENT, because it is
processed at a different stage than main_loop.events. In order to queue
into main_loop.events, we need to go through the threadsafe
loop_schedule(), which queues into main_loop.thread_events and
eventually main_loop.fast_events. _Then_ it is safe to directly queue
into main_loop.events.
This makes it more likely that the event is treated as K_EVENT.