Interface OrderedAdapter

Optional adapter mixin for sibling z-order.

Both methods are optional. Reorder ops and useReorder no-op when either is absent — adopt z-order incrementally without breaking existing adapters.

Convention: array order IS z-order. Index 0 is the bottom of the stack, the last index is the top. Hit-testing should iterate the returned list in REVERSE (top to bottom). Render layers iterate FORWARD (bottom to top).

For groups, parentId may be a group id; the group adapter routes getChildren/setChildOrder to the group's members[] array.