Vue multiple list example throws StructuredClone errors

Applicable [here](https://experimental--5fc05e08a4a65d0021ae0bf2.chromatic.com/?path=/story/vue_sortable-multiple-lists--basic-setup)

To trigger the error, reorder item A1 to column B and move it back to column A, as soon as the second drag starts the error gets triggered

Making the `items` a `shallowRef` instead of a `ref` works

Start with the linked Vue Storybook example and reproduce the error by moving item A1 to column B and back to column A. Compare the behavior when items uses ref versus shallowRef, then trace the StructuredClone failure during the second drag. Done means the sequence completes without the error while preserving multi-list dragging.

Written by the indexing model from the issue text.