Add comprehensive ANN benchmarking suite

Extend benchmarks-ann/ with results database (SQLite with per-query detail
and continuous writes), dataset subfolder organization, --subset-size and
--warmup options. Supports systematic comparison across flat, rescore, IVF,
and DiskANN index types.

benchmarks-ann/.gitignore

@@ -1,2 +1,8 @@
 *.db
+*.db-shm
+*.db-wal
+*.parquet
 runs/
+
+viewer/
+searcher/