DataTable: TableBulkActions re-creates a MutationObserver on document.body every render #8715

Part of #8676 (P0).

Problem

DataTable/TableBulkActions.tsx:58-63 runs a useLayoutEffect with no dependency array. Each run calls observeFocusedAction() (:48), which creates a new MutationObserver on element.ownerDocument.body with {childList: true, subtree: true}. So every render of the bulk actions bar disconnects and rebuilds a document-wide observer.

onFocusCapture already calls observeFocusedAction() when focus moves into the bar, so the per-render effect is redundant for focus tracking.

Verified in code but not measured.

Proposed fix

Do one of the following:

• Drop the per-render call and rely on onFocusCapture, keeping only an unmount cleanup that disconnects the observer.
• Give the effect a dependency such as selectedCount.

recoverFocus() must still run when an action unmounts while focused.

Acceptance criteria

• Re-rendering TableBulkActions with unchanged props doesn't create a new MutationObserver.
• Focus recovery still works when the focused bulk action is removed. Add or keep a test for this.
• Unmounting disconnects the observer.