fix(gpu_hnsw): bound visited-bitmap VRAM via nq-chunked layer-0 search

[devin-ai-integration]

Summary

Fixes a device-memory OOM in GPU HNSW search caused by the grow-only search scratch. The per-slot visited bitmap is nq * ceil(N/32) * 4 bytes and, once grown, is never released. Under high search concurrency (one bitmap per concurrent pool slot) on large segments this exhausts VRAM — 16 concurrent batch=512 searches on a ~538M-row segment grew the pool to ~97 of ~98 GB and OOM'd every subsequent allocation (surfaced during the v90 Blackwell validation).

Fix: process queries in nq-chunks so the bitmap is bounded regardless of batch size / concurrency. No kernel changes — every per-query buffer (queries, entry points, neighbors, distances, visited bitmap, BF worklist) is already indexed by the chunk-local block index (blockIdx.x / worklist entry), so the host just launches per chunk against caller-offset base pointers. Queries are independent in HNSW, so chunking is result-invariant.

chunk_nq = gpu_hnsw_bitmap_chunk(nq, N)   // = nq when the batch fits the cap
for c0 in 0, chunk_nq, 2*chunk_nq, ...:
    cnq  = min(chunk_nq, nq - c0)
    q0   = d_layer0_queries + c0*dim       // int8 on DP4A path, else fp32
    fq0  = d_queries        + c0*dim       // fp32, upper-layer descent
    ep0  = d_entry_points   + c0
    nb0  = d_neighbors      + c0*k
    ds0  = d_distances      + c0*k
    <upper-layer greedy descent>(fq0, ep0, cnq)
    memset(visited_bitmap, 0, calc_visited_bitmap_size(cnq, N))
    layer0_beam_search<<<cnq, ...>>>(q0, ep0, nb0, ds0, cnq, ...)   // + per-chunk BF fallback

• gpu_hnsw_bitmap_chunk(nq, N) derives the per-launch query count from a VRAM cap (default 256 MiB, env-tunable via GPU_HNSW_BITMAP_MB, or GPU_HNSW_BITMAP_BYTES for byte-precise control in tests). Always in [1, nq]; returns nq when the batch already fits, so small segments run a single pass — behavior identical to before.
• GpuHnswSearchScratch::ensure() now sizes the bitmap for one chunk (bm_nq) instead of the full batch — this is the actual allocation that no longer grows unbounded. The cap only ever lowers the chunk vs. what ensure() sized, so the buffer is always large enough for the launched chunk.
• The shared-memory sizing / ef finalization block is query-count independent and is hoisted to run once before the chunk loop; the cudaFuncSetAttribute high-water tracking (per <DataT,QueryT,USE_DP4A,HAS_FILTER>) is unchanged.

All existing semantics preserved: FP32 / FP16 / BF16 / generic-INT8 / native-INT8-DP4A, L2 / IP / COSINE, filtered search (two-tier beam + alpha gate + per-query BF fallback), up-front brute force, valid IDs / sentinels, CPU-HNSW parity.

Parity coverage lands with the knowhere re-vendor (this faiss tree only builds inside knowhere): a [gpu_hnsw_chunk_parity] test runs each search twice (default cap = single pass vs. GPU_HNSW_BITMAP_BYTES forced tiny = many chunks) and asserts identical ids/distances across fp32 & int8-DP4A, unfiltered / mid-ratio filtered / up-front-BF (99%) for L2/IP/COSINE.

Related PRs

• knowhere re-vendor (+ parity test): feat(gpu_hnsw): re-vendor faiss GPU tree (nq-chunked visited bitmap, OOM fix) knowhere#10
• milvus pin bump: follows once knowhere merges.

Link to Devin session: https://6sense.devinenterprise.com/sessions/55dab0bd33c346df99b5818b30059342
Requested by: @premal

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[devin-ai-integration]

Superseded by the current gpu-hnsw stack (#11). The nq-chunked visited-bitmap OOM fix is included there. Closing; branch retained.