Method addEvent

* Low-level event append. For a known stateful `type` it applies the matching * snapshot mutation then appends; for non-mutating types it appends only. * Used internally by the typed setters; exposed for the tracing worker's * `task.custom` observability needs and for callers that kno

(
        taskId: string,
        type: string,
        payload: Record<string, unknown>,
        opts?: TaskMutationOptions
    )