perf(datagrid): cache column index lookup in selection drawing

[desperadoxhy]

问题

GridSelectionOverlay.frame 与 DataGridRowView.drawCellSelectionFill 每次 draw 对每个选中列调 DataGridView.tableColumnIndex(内部 tableView.column(withIdentifier:) 线性扫 tableColumns)。拖选 N 列 × 每行 × 多次 draw = O(N²),列多 + 选区大时滚动卡。

方案

在 TableViewCoordinator 缓存 columnIndexByDataIndex: [Int: Int],draw 时直接读。失效点:列增删/重排/隐藏(reconcile 后)、用户拖列(tableViewColumnDidMove)、schema 变化、teardown。列宽不影响 index,不在 resize 时失效。

验证

• BUILD SUCCEEDED
• ColumnIndexCacheTests 5 个:解析正确、重复调用稳定、列重排后失效反映新顺序、schema 变化失效、越界返回 nil
• 既有 CellSelectionTests + DataGridSelectionTests 全过

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[desperadoxhy]

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