[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

从 12.batcharray.ipynb 开始，在决定扩展现有内容还是添加 12b.listarray.ipynb 之前，先检查现有的 BatchArray 教程结构。涵盖列出的 ListArray 操作、使用 dataclass schema 的 CTable、与 Arrow 的往返转换，以及关于 ListArray 与 BatchArray 的选择指导；当每个请求的工作流都有一个可运行的完整示例时，即视为完成。

由索引模型根据 Issue 内容生成。