feat(select): Ctrl+click picks one segment of a polyline

Ctrl+click on a polyline selects the segment under the cursor (again to
drop it); several segments can be picked. Only the picked segments are
highlighted, and MOVE, ROTATE, SCALE and MIRROR then change just their
vertices, so the neighbouring segments stretch to follow. Any other
selected objects are transformed whole as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scene/selection.rs

@@ -165,6 +165,7 @@ impl Scene {
 
     pub fn deselect_all(&mut self) {
         self.selected_constraint = None;
+        self.sub_segments.clear();
         if self.selected.is_empty() {
             return;
         }