perf(grid): eliminate eager List<String> allocation on every pointer-move during drag selection #1348

Summary

In lib/features/workspace/grid/grid_pointer_selection.dart (lines 12–25, 142–154), during mouse-drag selection (_isDragSelecting), whenever the pointer moves and crosses a cell coordinate boundary:
_notifySelectionAndFocus() extracts all cell values in the selection bounding box:

final values = <String>[];
for (var r = minRow; r <= maxRow; r++) {
  final row = _sortedRows[r];
  for (var c = minCol; c <= maxCol; c++) {
    values.add(row[c].toString());
  }
}
widget.onSelectionChanged?.call(_selection, values);

For a 100-row × 10-column selection box, this allocates and populates a new 1,000-element List<String> on every pointer-move event (up to 60–120 times per second during dragging).
Meanwhile, ResultsTab already debounces selection calculations by 60 ms (_calcDebounceTimer). The intermediate lists allocated during active dragging are discarded immediately, causing unnecessary GC pressure and transient frame drops during gestures.

Proposed Solution

1. Separate coordinate selection updates from value extraction:
   • Provide lightweight coordinate bounds in onSelectionChanged without eagerly extracting strings.
   • Defer string extraction until drag ends (pointer-up) or until the debounced calculation timer fires.
2. Only query the selected cell string values when actually calculating stats or copying to clipboard.

Acceptance Criteria

• Dragging selection across large cell ranges does not allocate intermediate List<String> objects on every pointer-move event.
• Selection highlighting and live coordinate bounds update smoothly at 60/120 Hz.
• Selection calc metrics in ResultsTab remain accurate and properly updated.