Handle generator exhaustion

[sneakers-the-rat]

A draft of fixing #147 - but we should not merge this, in working on this i realized that our stopping logic in general needs some work, related to #206 as well.

So this was part of the "see if fable can actually do stuff" test run, the initial commit is its attempt and called it good. clearly that is inadequate.

mine adds handling exhaustion as an event but is similarly hacky - the things we need are

• Deprecate implicit runner inits #247 - better control over initialization condition, we need a general way to shut it down and exist from whatever run is happening.
• move the event handling into the scheduler, so the scheduler is the thing that says "can't run anymore bub" - so rather than a million different handlers in the different runners, if the scheduler comes across an exhaustion event, it yields the remaining nodes that can run, and then future calls to the ready iterators raise an exception that says "can't run no more"
• more coherent hook handling in general across the runners - we could do with a bit of cleanup, they are sort of diverging from one another and the hook methods aren't actually usable because of that divergence. each of the runners does have a different design, but still it shouldn't be this hard to handle an event.

opening this up as a draft, but we should fix the other things first, and i'll need to think about how we want to handle this in the scheduler.

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📚 Documentation preview 📚: https://noob--248.org.readthedocs.build/en/248/