Cherry-picked upstream fixes and performance improvements:

- Performance: Cache-line-padded striped locks for better concurrency
- Fix: Heap-buffer-overflow in HNSW search
- Fix: Stale vector references in refine_() during update
- Fix: Duplicate neighbors in reverse links
- Fix: UB in index_dense::add pre-allocation
- Fix: Hash set tombstone corruption in add/remove cycles
- Fix: Postfix iterator semantics for member/candidate iterators