perf(datagrid): serve row count from cache to smooth scrolling

[desperadoxhy]

问题

NSTableView 在布局、滚动时多次调 numberOfRows，每次都重新构造行集合(tableRowsProvider()，struct 拷贝 + refcount churn)。滚大表时开销累积。

方案

numberOfRows 改用已维护的 cachedRowCount。逐一核对了 6 条行数变更路径(初始加载、增删行、全量替换、撤销重做)，确认每条都在 reloadData/insertRows/removeRows 之前调用了 updateCache()，cachedRowCount 无滞后窗口。

验证

• BUILD SUCCEEDED
• TableViewCoordinatorRowCountCacheTests 覆盖 cachedRowCount 与实际行数在加载、追加、删除、全量替换下的一致性

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