KiteSelectionHandlePainter

Draws one selection boundary marker ("thumb") for KiteDocView.

Called once per edge inside the page's draw pass. x is the boundary's horizontal position; top/bottom are the boundary line's vertical extent, all in page-slot pixels. Size the marker against bottom - top (the line height) so it scales with the text through thumbnails and deep zoom, like KiteSelectionHandleDefaults.CaretAndDot does.

Drawing does not move the grab target. A thumb is dragged by pressing near the boundary line, not near the shape you paint, so a marker drawn far from its boundary still answers to the boundary. Keep it close to the line and the two agree.