Add comprehensive ANN benchmarking suite (#279)

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/results_schema.sql

@@ -0,0 +1,76 @@
+-- Comprehensive results schema for vec0 KNN benchmark runs.
+-- Created in WAL mode: PRAGMA journal_mode=WAL
+
+CREATE TABLE IF NOT EXISTS runs (
+  run_id          INTEGER PRIMARY KEY AUTOINCREMENT,
+  config_name     TEXT    NOT NULL,
+  index_type      TEXT    NOT NULL,
+  params          TEXT    NOT NULL,   -- JSON: {"R":48,"L":128,"quantizer":"binary"}
+  dataset         TEXT    NOT NULL,   -- "cohere1m"
+  subset_size     INTEGER NOT NULL,
+  k               INTEGER NOT NULL,
+  n_queries       INTEGER NOT NULL,
+  phase           TEXT    NOT NULL DEFAULT 'both',
+    -- 'build', 'query', or 'both'
+  status          TEXT    NOT NULL DEFAULT 'pending',
+    -- pending → inserting → training → querying → done | built | error
+  created_at_ns   INTEGER NOT NULL    -- time.time_ns()
+);
+
+CREATE TABLE IF NOT EXISTS run_results (
+  run_id              INTEGER PRIMARY KEY REFERENCES runs(run_id),
+  insert_started_ns   INTEGER,
+  insert_ended_ns     INTEGER,
+  insert_duration_ns  INTEGER,
+  train_started_ns    INTEGER,       -- NULL if no training
+  train_ended_ns      INTEGER,
+  train_duration_ns   INTEGER,
+  build_duration_ns   INTEGER,       -- insert + train
+  db_file_size_bytes  INTEGER,
+  db_file_path        TEXT,
+  create_sql          TEXT,           -- CREATE VIRTUAL TABLE ...
+  insert_sql          TEXT,           -- INSERT INTO vec_items ...
+  train_sql           TEXT,           -- NULL if no training step
+  query_sql           TEXT,           -- SELECT ... WHERE embedding MATCH ...
+  k                   INTEGER,       -- denormalized from runs for easy filtering
+  query_mean_ms       REAL,          -- denormalized aggregates
+  query_median_ms     REAL,
+  query_p99_ms        REAL,
+  query_total_ms      REAL,
+  qps                 REAL,
+  recall              REAL
+);
+
+CREATE TABLE IF NOT EXISTS insert_batches (
+  batch_id        INTEGER PRIMARY KEY AUTOINCREMENT,
+  run_id          INTEGER NOT NULL REFERENCES runs(run_id),
+  batch_lo        INTEGER NOT NULL,   -- start index (inclusive)
+  batch_hi        INTEGER NOT NULL,   -- end index (exclusive)
+  rows_in_batch   INTEGER NOT NULL,
+  started_ns      INTEGER NOT NULL,
+  ended_ns        INTEGER NOT NULL,
+  duration_ns     INTEGER NOT NULL,
+  cumulative_rows INTEGER NOT NULL,   -- total rows inserted so far
+  rate_rows_per_s REAL    NOT NULL    -- cumulative rate
+);
+
+CREATE TABLE IF NOT EXISTS queries (
+  query_id          INTEGER PRIMARY KEY AUTOINCREMENT,
+  run_id            INTEGER NOT NULL REFERENCES runs(run_id),
+  k                 INTEGER NOT NULL,
+  query_vector_id   INTEGER NOT NULL,
+  started_ns        INTEGER NOT NULL,
+  ended_ns          INTEGER NOT NULL,
+  duration_ms       REAL    NOT NULL,
+  result_ids        TEXT    NOT NULL,  -- JSON array
+  result_distances  TEXT    NOT NULL,  -- JSON array
+  ground_truth_ids  TEXT    NOT NULL,  -- JSON array
+  recall            REAL    NOT NULL,
+  UNIQUE(run_id, k, query_vector_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_runs_config  ON runs(config_name);
+CREATE INDEX IF NOT EXISTS idx_runs_type    ON runs(index_type);
+CREATE INDEX IF NOT EXISTS idx_runs_status  ON runs(status);
+CREATE INDEX IF NOT EXISTS idx_batches_run  ON insert_batches(run_id);
+CREATE INDEX IF NOT EXISTS idx_queries_run  ON queries(run_id);