Pure, typed actor-behavior primitives.
[Protocol] is stable public destination identity (Addr plus Msg). A
[Behavior] separately owns state and folds its complete [Behavior::Event]
algebra into exactly [Actions]: sends, fresh creations, and its next
behavior or termination. A protocol is not a behavior, and Behavior is not
a Protocol supertrait. Higher capabilities extend internal event and
effect algebras while transparent wrappers preserve [Behavior::Protocol].