Faster KNN index builds in Manticore

Building KNN indexes just got a whole lot faster in Manticore :rocket:

With parallel KNN index builds, rebuilding an HNSW graph for 1M vectors dropped from 8 minutes to just 39 seconds on a 16-core machine — up to 11× faster.

The improvement applies to chunk saves, OPTIMIZE merges, auto-optimize, and ALTER TABLE … REBUILD KNN.