Add an InstanceStarted event for atomic start-time wiring

Add a typed InstanceStartedEvent dispatched inside the start transaction, after the instance is saved (so it has its id) and its variables are seeded, but before the start token is placed and the workflow runs.

This is the extension point for wiring an external entity to a process atomically with its start: a subscriber links the entity to the instance using the now-known instance id, the link commits with the start (a subscriber that throws rolls the start back), and it is in place before a synchronous run. It is the idiomatic, decoupled alternative to passing a callback into start(); a plain entity insert hook is too early, since it fires before the variables are seeded, so a subscriber could not tell which entity the run is for.