Kinds were already an enum with a total parse and an Unknown case; triggers and
emits were bare strings, so a host matched "on_event" by hand with no compiler
help and an unrecognised value read like any other. TriggerType, TriggerEvent
and FieldEmit now get the same treatment, and fires_on answers the question a
host actually has. Cadence stays the host's obligation: only it knows what has
already run.