Add ANN search support for vec0 virtual table (#273)

Add approximate nearest neighbor infrastructure to vec0: shared distance
dispatch (vec0_distance_full), flat index type with parser, NEON-optimized
cosine/Hamming for float32/int8, amalgamation script, and benchmark suite
(benchmarks-ann/) with ground-truth generation and profiling tools. Remove
unused vec_npy_each/vec_static_blobs code, fix missing stdint.h include.

benchmarks-ann/schema.sql

@@ -0,0 +1,35 @@
+-- Canonical results schema for vec0 KNN benchmark comparisons.
+-- The index_type column is a free-form TEXT field. Baseline configs use
+-- "baseline"; index-specific branches add their own types (registered
+-- via INDEX_REGISTRY in bench.py).
+
+CREATE TABLE IF NOT EXISTS build_results (
+  config_name  TEXT NOT NULL,
+  index_type   TEXT NOT NULL,
+  subset_size  INTEGER NOT NULL,
+  db_path      TEXT NOT NULL,
+  insert_time_s REAL NOT NULL,
+  train_time_s REAL,            -- NULL when no training/build step is needed
+  total_time_s REAL NOT NULL,
+  rows         INTEGER NOT NULL,
+  file_size_mb REAL NOT NULL,
+  created_at   TEXT NOT NULL DEFAULT (datetime('now')),
+  PRIMARY KEY (config_name, subset_size)
+);
+
+CREATE TABLE IF NOT EXISTS bench_results (
+  config_name  TEXT NOT NULL,
+  index_type   TEXT NOT NULL,
+  subset_size  INTEGER NOT NULL,
+  k            INTEGER NOT NULL,
+  n            INTEGER NOT NULL,
+  mean_ms      REAL NOT NULL,
+  median_ms    REAL NOT NULL,
+  p99_ms       REAL NOT NULL,
+  total_ms     REAL NOT NULL,
+  qps          REAL NOT NULL,
+  recall       REAL NOT NULL,
+  db_path      TEXT NOT NULL,
+  created_at   TEXT NOT NULL DEFAULT (datetime('now')),
+  PRIMARY KEY (config_name, subset_size, k)
+);