DataTable: toggling one row's selection re-renders every cell #8716

Part of #8676 (P1). Measure with #8714 before and after.

Problem

DataTable/useTable.ts (createRow, about :292-318) builds new row and cell objects on every render, with selected baked in at :309. TableRow (Table.tsx:264) and TableCell (:289) aren't memoized. Every row and cell therefore re-renders whenever any selection changes.

Measured: with 200 rows by 3 columns, mounting calls renderCell 600 times. Toggling one row's checkbox calls it 600 more times.

Passing a new data array also re-sorts and rebuilds every row (useTable.ts:177-181).

Proposed fix

• Cache row objects by source row plus selected and selectable, so unchanged rows keep their identity.
• Render rows through a memoized row component.
• Document that data and columns should be referentially stable.

The fix must work on React 18 and 19.

Acceptance criteria

• Toggling one row re-renders only that row's cells, plus the header and bulk-action state. Add a test that counts renderCell calls.
• Sorting, grouping, row selection and select-all behave the same as before.
• The PR reports before and after numbers from Add repeatable performance benchmarks for list, tree and button stress cases #8714.