reorderColumn method
Move column id to flat index toIndex in the current visible order.
Implementation
void reorderColumn(ColId id, int toIndex) {
final from = _columnOrder.indexOf(id);
if (from < 0) return;
final clamped = toIndex.clamp(0, _columnOrder.length - 1);
if (from == clamped) return;
_columnOrder.removeAt(from);
_columnOrder.insert(clamped, id);
_bump(rebuildLayout: true);
}