Add pixel-size fit and drag-to-reorder in the hierarchy

The aspect button now carries two offers, so it becomes a menu: fit one
side to the texture ratio, or set both sides to the exact pixel size as a
single undo step.

Dropping a row on the outer quarter of another row inserts it into that
gap instead of parenting into it, reordering among the siblings there.
Order is the stored order, so ReorderObjectCommand is a splice; equal z
falls back to tree order, which makes this the way to draw one object in
front of another. Rejected on container content like the other structural
edits.

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

src/core/model/hierarchy.ts

@@ -19,6 +19,11 @@ function childrenOf(tree: ObjectTree, parentId: string): SceneObject[] {
   return tree.objects.filter((o) => o.parentId === parentId);
 }
 
+/** Children of `parentId` (`''` = top level) in sibling order – what a reorder rearranges. */
+export function childIdsOf(tree: ObjectTree, parentId: string): string[] {
+  return childrenOf(tree, parentId).map((o) => o.id);
+}
+
 function parentOf(tree: ObjectTree, object: SceneObject): SceneObject | undefined {
   return object.parentId ? findObject(tree, object.parentId) : undefined;
 }