complete/uncomplete return a CompletionResult str subclass carrying
appended (decided with the write under the lock), so duplicate
concurrent calls fire one transition event; day-complete only
evaluates for an instance due today.
quest_completed and quest_uncompleted fire on actual transitions with
documented payloads; child_day_complete fires when a completion clears
the child's day; quest_missed is defined as Feature 11's contract.
Payloads documented in the README for automation authors.