Builds an AdaptiveSub from a function that subscribes to input deltas. Every state built from an input delta is written into the root at the step boundary, before Update, with the edges merged (see ActionState.mergeEdges); the consumed edges are cleared after Update, before the frame is forced. So Update reads each edge exactly once, and the next step starts from an empty edge set. The after-update clear goes through an internal member of the posting surface; it is not part of the public contract.