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 <[email protected]>

src/app/command_driver/transform.rs

@@ -24,6 +24,9 @@ impl OpenCADStudio {
         // component so each member keeps its own dimensions. Other
         // transforms retain their selected-object behavior.
         let pending = self.begin_undo(i, label, handles.len(), true);
+        let handles = self.tabs[i]
+            .scene
+            .transform_picked_segments(&handles, &transform);
         self.tabs[i].scene.transform_entities(&handles, &transform);
         self.apply_inferred_constraints(i, &handles);
         self.tabs[i].dirty = true;