Fix buffer invariants (#227)

After cleanup removes deleted nodes from the search buffer, its state
becomes invalid. Call sort() to restore the buffer’s invariants and
prepare it for the next round of iterator searches.

Author: ibhati

include/svs/index/vamana/iterator.h

@@ -295,6 +295,7 @@ template <typename Index, typename QueryType> class BatchIterator {
 
                     if constexpr (Index::needs_id_translation) {
                         buffer.cleanup();
+                        buffer.sort();
                     }
                 };