Center and Quadrant found nothing on a polyline's bulge segments: the
lightweight and heavy polylines carried no snap points for them, and a
polyline split into several wires kept its points on one piece whose box
the others' never reached. Each arc segment now carries its centre and
quadrants, hovering the segment offers its centre, and a wire's box
covers its snap points.
Fixes#1640
Co-Authored-By: Claude Opus 5.5 <[email protected]>