Fix #144385: "Reorder Mesh Spatially" broken with loose vertices

Fixes missing isolated vertices in `mesh_apply_spatial_organization` by
ensuring non-added vertices (ones not forming a face) are appended to
new_vert_order, preserving correct vertex ordering for meshes with
disconnected geometry.

Pull Request: https://projects.blender.org/blender/blender/pulls/144388

source/blender/blenkernel/intern/mesh.cc

@@ -858,6 +858,13 @@ void mesh_apply_spatial_organization(Mesh &mesh)
       }
     }
 
+    for (const int vert : IndexRange(mesh.verts_num)) {
+      if (!added_verts[vert]) {
+        new_vert_order.append(vert);
+        added_verts[vert].set();
+      }
+    }
+
     for (const int face_idx : local_group.faces) {
       new_face_order.append(face_idx);
     }