The runtime is sveltekitbook on npm. Two scaffolders ship with it — one for long-form pages, one for talks. The long-form one lays down the schema, the routes, the cover, and a contents page. None of it writes prose. It renders prose inside a frame that occupies the four slots on purpose. Your job is to fill the slots. The schema is the part worth keeping stable; the surface around it is yours to bend.