onRegister

Optional hook invoked by StreamMux.addTransformer immediately after the transformer is attached to the mux. Receives a limited handle that exposes only StreamEmitter.push — enough for the transformer to emit synthesized ProtocolEvents on any namespace it chooses (e.g. a deepagents SubagentTransformer fabricating lifecycle/messages/values events under a ["tools:<tool_call_id>"] namespace when a task tool starts).

Transformers that do not synthesize events can omit this hook.

The StreamEmitter handle is only safe to call from within StreamTransformer.process. Emitting from an unrelated async context (e.g. after process has returned, from a setTimeout, etc.) races with the mux's close/fail cycle and may land events in an already-closed log.