perf(grid): do not full re-sort on every staged cell edit #875

Summary

VirtualResultGrid._onStagingBufferChanged calls _updateSortedRows() whenever a sort column is active. Each cell edit therefore re-sorts the full result (O(N log N), isolate copy at N≥3000) and rebuilds a new row list. Dirty effectiveRows is also O(R) per notification.

Editing a sorted 5k-row grid on 120 Hz is a layout/CPU hitch per keystroke, not per sort click.

Scope

• Keep display rows stable while editing; apply staged values via getCellValue / overlay instead of re-sorting the whole cap.
• Re-sort on sort-column change, row insert/delete, or explicit user action — not on every notifyListeners.
• Avoid a second setState in _commitEdit if the staging listener already rebuilt.

Out of scope

• Staging buffer deep-copy of original rows (separate memory tradeoff).