Use a MouseButton enum for mouse button parameters

Mouse button parameters should use a typed MouseButton enum instead of raw integer IDs. This makes mouse event handlers and GameCanvas polling clearer, safer, and easier to discover through autocomplete.

Expected API shape:

Sub Canvas1_MouseDown(x As Double, y As Double, button As MouseButton)
    If button = MouseButton.Right Then
        # Show context menu
    End If
End Sub

If GameCanvas1.IsMouseDown(MouseButton.Left) Then
    # Primary button held
End If