#15024: Improve prefix sum in Lucene99HnswVectorsReader

[leng25]

Summary

This PR implements the optimization suggested in #15024, replacing the two-step prefix sum loop in Lucene99HnswVectorsReader with a single-pass accumulator variant that avoids redundant memory reads.

Before:

currentNeighborsBuffer[0] = dataIn.readVInt();
for (int i = 1; i < arcCount; i++) {
  currentNeighborsBuffer[i] = currentNeighborsBuffer[i - 1] + dataIn.readVInt();
}

After:

int sum = 0;
for (int i = 0; i < arcCount; i++) {
  sum += dataIn.readVInt();
  currentNeighborsBuffer[i] = sum;
}

This is a follow-up to #15027 by @yossev who proposed the same fix. Since that PR went stale (merge conflicts, formatting), I'm resubmitting with conflicts resolved, formatting fixed via ./gradlew tidy, and benchmark results included.

I found this while looking for a good first issue to learn the contribution process — happy to adjust anything based on feedback!

Benchmark Results

Benchmarks were run using luceneutil KNN benchmark (knnPerfTest.py).

Machine: Intel Core i5-10210U, 8 logical cores, ~15 GB RAM
Dataset: cohere-v3-wikipedia-en 1024d, 400k docs, 10k queries, 8-bit quantized, dot_product

Baseline:

recall  latency(ms)  netCPU  avgCpuCount    nDoc  topK  fanout  maxConn  beamWidth  quantized  visited  index(s)  index_docs/s  force_merge(s)  num_segments  index_size(MB)
 0.977        9.920   9.893        0.997  400000   100     100       64        250     8 bits     7955    486.32        822.50          437.90             1         2015.68

Candidate (this PR):

recall  latency(ms)  netCPU  avgCpuCount    nDoc  topK  fanout  maxConn  beamWidth  quantized  visited  index(s)  index_docs/s  force_merge(s)  num_segments  index_size(MB)
 0.977        9.861   9.833        0.997  400000   100     100       64        250     8 bits     7955    486.32        822.50          437.90             1         2015.68

Recall is identical. Results are from a single run so small differences may fall within normal measurement variance.

[kaivalnp]

LGTM

Looks like this improvement is in the range of noise for knnPerfTest.py, but is good-to-have anyways.

[kaivalnp]

nit: Would prefer this variable inside the if block below

[leng25]

Done, move inside the if block

[kaivalnp]

This entry is under 11.0.0 -- can you move it to 10.5.0? (I can help with merge + backport)

[leng25]

Move into 10.5.0, would appreciate help with the merge + backport

[kaivalnp]

LGTM