fix(stretch): hotspot stretch

 Fixed the hotspot-driven stretch path too.

  Root cause:

  - hotspot runtime geometry for instances is rebuilt in GroupScene.recomputeInstanceGeometryFromPrototype(...)
  - applyHotspotToRuntimeGeometry(...) stretches triangles in place
  - that breaks the DraftFaceStore color map for the same reason as the regular stretch tool: mutable triangle keys change after vertex mutation

  Patch:

  - core/src/main/kotlin/com/github/alfu32/sketch/model/GroupScene.kt:1788
      - after hotspot runtime geometry is applied, the temporary runtime lineStore and faceStore now call notifyExternalChange()
      - this rehashes:
          - face colors
          - face selection
          - line selection

Author: alferio

core/src/main/kotlin/com/github/alfu32/sketch/model/GroupScene.kt

@@ -1785,6 +1785,8 @@ class GroupScene(
                 vertexIdByHandle = vertexIdByHandle
             )
         }
+        lineStore.notifyExternalChange()
+        faceStore.notifyExternalChange()
 
         if (selectedSegmentIds.isNotEmpty()) {
             lineStore.getSegments().forEach { segment ->