Ping-pong turns on the last key, and stepped rows hold the key they last passed (#112)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/model/animationPreview.ts

@@ -30,19 +30,21 @@ export const PREVIEW_ROOT_ID = PLACED_ROOT_ID;
  * container through its instance's edits – the same route an edit in the Scene tab takes, so a row
  * aimed into a placed container needs nothing of its own (#109). `undefined` without a root. Past
  * the end of a `once-reset` pass nothing is written: the player has put the starting values back.
+ * `reverse` is ping-pong's way back, as `playheadAt` reports it.
  */
 export function previewHolder(
   project: Project,
   animation: AnimationAsset,
   frame: number,
+  reverse = false,
 ): ObjectHolder | undefined {
   const root = findAsset(project, animation.root);
   if (!root) return undefined;
   const reset = playbackOf(animation) === 'once-reset' && frame >= frameCount(animation);
   const holder = root.type === 'scene' ? sceneCopy(root) : placedHolder(root);
   for (const track of reset ? [] : animation.tracks) {
-    if (root.type === 'scene' && track.target === '') writeScene(holder, track, frame);
-    else writeObject(project, holder, previewObjectId(root, track.target), track, frame);
+    if (root.type === 'scene' && track.target === '') writeScene(holder, track, frame, reverse);
+    else writeObject(project, holder, previewObjectId(root, track.target), track, frame, reverse);
   }
   return holder;
 }
@@ -72,10 +74,10 @@ function placedHolder(root: Asset): ObjectHolder {
 }
 
 /** A row on the scene itself: its value goes into the copy's own properties. */
-function writeScene(holder: ObjectHolder, track: AnimationTrack, frame: number): void {
+function writeScene(holder: ObjectHolder, track: AnimationTrack, frame: number, reverse: boolean): void {
   const def = animatableDefs('scene').find((d) => d.key === track.property);
   if (!def) return;
-  const value = trackWriteAt(track, def.kind, frame, holder.props[track.property] ?? def.default);
+  const value = trackWriteAt(track, def.kind, frame, holder.props[track.property] ?? def.default, reverse);
   if (value !== undefined) holder.props[track.property] = value;
 }
 
@@ -85,12 +87,13 @@ function writeObject(
   id: string,
   track: AnimationTrack,
   frame: number,
+  reverse: boolean,
 ): void {
   const ref = objectRef(project, holder, id);
   const asset = ref && findAsset(project, ref.object.assetId);
   const kind = asset && animatedKind(asset.type, track.property);
   if (!ref || !asset || !kind) return;
-  const value = trackWriteAt(track, kind, frame, baseOf(project, asset, ref.object, track.property));
+  const value = trackWriteAt(track, kind, frame, baseOf(project, asset, ref.object, track.property), reverse);
   if (value === undefined) return;
   if (isPositionKey(track.property))
     ref.setPosition({ ...ref.object.position, [track.property]: Number(value) });