Hit-test ListBox cells at a point

Add ListBox.CellAtPoint(x, y) so desktop applications can identify the visible cell under ListBox-relative pointer coordinates and obtain the pointer position within that cell. This supports precise interactions with custom-drawn cell content, such as changing the cursor only over an icon and distinguishing multiple clickable icons drawn in one cell.

The method returns an immutable ListBoxCellHit snapshot containing the displayed row, column, cell-relative X and Y coordinates, and the current rendered cell width and height. It returns Nothing when the point is not over a visible data cell.