In Merrill Feitell’s “The Cupcake Factory,” we bear witness to a moment between siblings that we know will become, for one of them, a searing memory. We know because it’s told as if the scene is already crystallizing as it unfolds, and with a weight that can only manifest. We talked to Feitell about the story, cupcakes, long projects, point of view, and shameful stacks of unread books.