More Guidance on Implementation

We should try to distill more about what we've learned about writing testable state machines into this. This includes areas like:

- moving IO out of the state machines into a "routing layer"
- routing between internal state machines
- "scheduler" state machines for scheduling/receiving network messages
- feeding output messages from a state machine back into itself
- state machines returning one or many output events
- event and error data structure design
- etc.

Some of this might be more language specific, but maybe that's ok - it would be valuable to have VDD guidance for particular languages ...

No file, test, or documentation entry point is identified. Review the existing VDD guidance, then scope the requested material around routing layers, scheduler state machines, output events, and event or error data structures. Done means adding coherent guidance, potentially with language-specific examples.

Written by the indexing model from the issue text.