fix(snap): arc segments of a polyline offer their centre and quadrants

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 <noreply@anthropic.com>

src/entities/polyline.rs

@@ -493,7 +493,13 @@ fn centerline_metadata_2d(
 
 impl RenderConvertible for Polyline2D {
     fn to_render(&self, document: &codec::CadDocument) -> Option<RenderEntity> {
-        Some(tessellate_polyline2d(self, document.header.fill_mode))
+        let mut render = tessellate_polyline2d(self, document.header.fill_mode);
+        // Arc segments offer their centres and quadrants.
+        if self.vertices.iter().any(|v| v.bulge != 0.0) {
+            let entity = codec::EntityType::Polyline2D(self.clone());
+            render.snap_pts.extend(crate::entities::curve::arc_segment_snaps(&entity));
+        }
+        Some(render)
     }
 }