Fix: Avoid duplicate neighbor slots in HNSW reverse links (#699)

* Fix: Avoid duplicate neighbor slots in HNSW reverse links
* Improve: Use `std::find` instead of `std::all_of` for neighbor presence check in `index_gt::form_reverse_links_`
* Fix: Use `std::find_if` for close_header neighbor check

Author: yoonseok-kim

include/usearch/index.hpp

@@ -3901,7 +3901,10 @@ class index_gt {
             // If `new_slot` is already present in the neighboring connections of `close_slot`
             // then no need to modify any connections or run the heuristics.
             if (close_header.size() < connectivity_max) {
-                close_header.push_back(new_slot);
+                if (std::find_if(close_header.begin(), close_header.end(),
+                                 [new_slot](compressed_slot_t slot) { return slot == new_slot; }) == close_header.end()) {
+                    close_header.push_back(new_slot);
+                }
                 continue;
             }