Stage 3 — authoring the motion

A composed scene is a set of layered frames that render identical to the design and don't move yet. This stage makes them move — and the gap between "slideshow" and "ad" lives entirely in the choices made here: which elements move, how, and what carries between frames. The design is authored as a scene throughout; every edit is a patch_scene that writes a new scene document (sceneRef moves — always edit the latest).

The rule under everything: motion draws the eye, so it is spent, not sprayed. A frame where everything animates at once reads as noise. Pick the focal element per beat, move it, let the rest support.

Motion in any scene, not only after an import