add per-state hook for handling discarded events

Any event type that is currently unexpected will be discarded. We add a hook that can be installed via the state designer DSL so that discarded events can be processed. This is important for user code in order to trigger compensating actions after the log merge has invalidated some previous states that were acted upon.

Start by reading the state designer DSL and the event-handling path where unexpected events are discarded. Trace how per-state behavior is currently represented and check existing tests around discarded or unexpected events, if present. Done means the DSL can install a per-state hook and discarded events reach it so user code can process them.

Written by the indexing model from the issue text.