perf(vamana): look up PQ build distances in a per-search table

[wombatu-kun]

What

The PQ-guided Vamana build (DiskAnnBuildDistance::ProductQuantized, the default) scores every candidate of a build search as a sum of m lookups centroid_distances[sub][candidate_code][query_code] in an m x 256 x 256 table (8 MiB at m = 32, 60 MiB at m = 240). The query, the node being inserted, is fixed for the whole search, so these lookups walk table columns with a 1 KiB stride, and lookups for different candidate codes never share a cache line.

This PR copies, once per search, the m rows selected by the query's codes into a per-worker m x 256 buffer (32 KiB at m = 32, 240 KiB at m = 240) and scores all candidates from it. The table is symmetric, so every value and the summation order stay the same: graphs and index files are bit-identical, and every index file in the benchmark below has the same SHA-256 before and after the change. The build memory estimate now includes the buffers.

Why it is faster

Before the change one search could touch up to m x 16 KiB of the table (512 KiB at m = 32, 3.75 MiB at m = 240), and twelve concurrent searches up to 6 MiB and 45 MiB, competing with the graph, codes and vectors for 256 KiB of L2 per core and a 9 MiB L3. After it a search reads only its m KiB buffer. This is consistent with the gain growing with m in the benchmark below (1.41x at m = 32, 3.00x at m = 240). The cache behaviour is reasoned from the access pattern; I did not collect cache-miss counters.

Benchmark

i7-8750H (6 cores / 12 threads, 9 MiB L3), performance power profile, 12 threads, DiskAnnBuildParams::default() with 8-bit PQ (m = d / 4). "Vamana passes" is the time of both graph passes, "index write" the time of the whole write_diskann_index_with_stats. SIFT1M values are the mean of two runs each, interleaved main, PR, PR, main (spread under 1%); GIST1M is one run each.

Dataset	Stage	main (s)	this PR (s)	Change
SIFT1M (d = 128, m = 32)	Vamana passes	210.2	148.9	-29.2% (1.41x)
SIFT1M (d = 128, m = 32)	Index write	224.0	162.6	-27.4% (1.38x)
GIST1M (d = 960, m = 240)	Vamana passes	2746.2	914.6	-66.7% (3.00x)
GIST1M (d = 960, m = 240)	Index write	2839.0	1008.4	-64.5% (2.82x)