Add a fast path that copies the first segment's index file as the merge base and only merges the tail segments into it. Limited to streaming indexes (HNSW, HNSW_RABITQ, FLAT) with matching index_type + quantize_type and no filter; IVF/VAMANA always rebuild (their Merge is dump-then-reopen and would drop the base docs).
Also, fix the incorrect concurrency of the compaction task.
Jalin Wang