Invoke and actors

invoke starts an actor when a state is entered. MachineLogic registers services:

• service(name, |ctx, input| -> Value) — a synchronous service, run to completion; its result is the xstate.done.actor.<id> output, an error becomes xstate.error.actor.<id>.
• async_service(name, |ctx, input| -> future) — runs on async interpreters.
• machine_service(name, machine) — a child chart as an actor.

onDone / onError transition on the done / error events; id and systemId address the actor for sendTo / sendParent. In memory the engine emits the v5 event spellings (xstate.done.actor.*, xstate.error.actor.*) and accepts the v4 spellings on input; snapshots and the structure hash use the v4 spellings (ADR-0001, ADR-0003).

The async runtime (tokio) runs child machines as tasks and supports spawnChild / stopChild. The core engine only knows “start invoke X with input Y” and “deliver the done/error event” — the driver decides how to run it (ADR-0004).