`updatefound` event in service worker context seems unreliable

Currently we expose the service workers current registration as self.registration. In theory, the service worker can add an updatefound event handle on the registration to know when an event happens. This event in a service context, though, seems unreliable since it doesn't use ExtendableEvent. For example:

1. The updatefound event does not start the service worker if its been stopped. (AFAICT)
2. The updatefound event does not provide an event.waitUntil() method.

Should updatefound be usable in a service worker context? If so, should the browser start the worker to fire this event and should it be an ExtendableEvent? If its an ExtendableEvent, should waitUntil() delay the installation of the update?

Start by examining the service worker registration behavior described in the issue, focusing on updatefound, stopped workers, ExtendableEvent, and event.waitUntil(). Compare the intended behavior for each question and document a clear specification decision for whether the event starts the worker and delays update installation.