fix(assembly): guard onChanged against solver during document restore

During document restore, PropertyLinkList::Restore sets the Group property
on AssemblyObject, triggering onChanged → updateSolveStatus → solve →
validateNewPlacements. At this point joints reference objects that haven't
been restored yet, causing a null pointer dereference (SIGSEGV).

Add an isRestoring() guard to skip solver invocation during restore,
matching the existing pattern in AssemblyLink::onChanged().

src/Mod/Assembly/App/AssemblyObject.cpp

@@ -139,6 +139,11 @@ App::DocumentObjectExecReturn* AssemblyObject::execute()
 
 void AssemblyObject::onChanged(const App::Property* prop)
 {
+    if (App::GetApplication().isRestoring()) {
+        App::Part::onChanged(prop);
+        return;
+    }
+
     if (prop == &Group) {
         updateSolveStatus();
     }