Part (a) of the animation rework: model, commands, saving, runtime and nodes.
- AnimationAsset holds a root (sprite, text, container or scene) and rows; a row keys one
property of the root or of an object inside it, keys sit on whole frames.
- The last key decides the length (its frame counts as a frame of its own). The playhead runs
between frames, so numbers and colors blend every tick even at 10 fps; everything else holds.
- Playback gains `once-reset`, which puts back what was there before the start. A row can be
relative, adding to where its target stood.
- Play / Pause / Stop animation act on an object; each instance plays on its own, the later
start wins on a shared property, and a behavior writing the same property still writes last.
- A sprite no longer wears an animation through `texture`. "Create animation from texture"
builds a sprite plus a texture row keyed frame by frame instead.
- The Animation tab shows a placeholder until the timeline editor (part b) lands.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>