fix: prevent unnecessary key updates in index_gt::update

Author: yoonseok-kim

include/usearch/index.hpp

@@ -2987,7 +2987,8 @@ class index_gt {
             }
             form_reverse_links_(metric, updated_slot, closest_view, value, level, context);
         }
-        updated_node.key(key);
+        if (updated_node.key() != key)
+            updated_node.key(key);
 
         // Normalize stats
         result.computed_distances = context.computed_distances - result.computed_distances;