fix seen of searcher not been clear when lower_search

Author: miamia0

Diff for: src/hnsw/hnsw_const.rs

@@ -391,6 +391,8 @@ where
             index: new_index,
             distance,
         };
+        searcher.seen.clear();
+        searcher.seen.insert(candidate.index);
         // Insert the index of the nearest neighbor into the nearest pool for the next layer.
         searcher.nearest.push(candidate);
         // Insert the index into the candidate pool as well.