applyAction
fun applyAction(state: <Error class: unknown class>, action: Action, payload: <Error class: unknown class> = JsonNull): ApplyResult
Runs action against state and returns the settled state plus the events it emitted.
Pure: no I/O, no clock, no store. The input state is never mutated — the interaction works on a private copy. An ActionError is returned in the result, never thrown, because a failed step is an ordinary outcome of an interaction rather than an exception the caller must guard a gesture callback against.
payload is what {"$event": "/pointer"} reads inside the action — the runtime data the interaction carried.