🙏希望可以针对合并后的单元格设置对齐样式

### Describe the feature / 功能描述

希望可以对合并后的单元格设置对齐方式, 例如横向:左对齐,居中,右对齐, 纵向: 顶部对齐, 居中,底部对齐
例如下图

### Design the API / API 设计

| name | type | default | description |
| ---- | ---- | ------- | ----------- |
| - | - | - | - |

### Are you willing to contribute? / 是否愿意参与贡献？

✅ Yes / 是

Start by reading the S2 code that handles merged cells and cell styling; the issue does not identify a file, test, or API entry point. Define how horizontal and vertical alignment should apply to merged cells, then verify the behavior with coverage for left, center, right, top, and bottom alignment.

Written by the indexing model from the issue text.