[DOC] Tutorial: variable-length columns (ListArray and BatchArray)

12.batcharray.ipynb covers BatchArray basics but there is no tutorial for ListArray — the newer, more flexible varlen column type. Users working with nested or ragged data (e.g. GPS waypoints, tags, embeddings of varying length) have no worked example to follow.

Suggested work: Extend 12.batcharray.ipynb or add 12b.listarray.ipynb covering:
- Creating a ListArray with item_spec
- Appending, extending, and fancy-indexing
- Embedding a ListArray as a column inside a CTable (using the dataclass schema API)
- Arrow round-trip (from_arrow / to_arrow)
- When to prefer ListArray over BatchArray

Start with 12.batcharray.ipynb and inspect the existing BatchArray tutorial structure before choosing whether to extend it or add 12b.listarray.ipynb. Cover the listed ListArray operations, dataclass-schema CTable usage, Arrow round-tripping, and the ListArray-versus-BatchArray guidance; done means each requested workflow has a runnable worked example.

Written by the indexing model from the issue text.