A one-shot promise whose settlement is controlled externally.
Use this when one part of a workflow must wait for an event that is owned
elsewhere—for example, a queued mutation waiting for the worker that will
push it. Deferred is awaitable because it implements PromiseLike, and
.promise is available when a concrete Promise is required.
The first call to resolve or reject wins; later calls are ignored. This
class deliberately does not provide cancellation, reset, or notification
semantics. It models exactly one eventual outcome.
