🙏 明细表-多表头能不能支持自定义合并

### 🖋 Description

明细表的多表头能够支持自定义合并列头单元格, 如下图

### 🏞 What problem does this feature solve

目前无法自定义合并列头单元格

Start by reviewing the existing detail-table multi-header handling and how header cells are represented. Define the customization needed for merged column-header cells from the attached example, then verify that the resulting headers render correctly without breaking existing multi-header behavior.

Written by the indexing model from the issue text.