Replace full-table scan via vec_distance_cosine() + ORDER BY LIMIT with
sqlite-vec's native MATCH + k = ? KNN operator. Keep vec_distance_cosine()
in the SELECT so score = 1 - dist preserves the existing cosine [0,1]
semantics the downstream merge pipeline depends on.
Fixes#69666.
Benchmark on 10,827 chunks, 4096-dim embeddings:
- Before (full scan): ~8490 ms/query
- After (KNN + join): ~50 ms/query
No behavioral changes: returned ids and ordering are identical to the
previous query on all tested queries. The LIMIT ? binding is replaced by
k = ? which caps sqlite-vec's candidate set to the same count.
7cd051d7f7