Add IVF index for vec0 virtual table

Add inverted file (IVF) index type: partitions vectors into clusters via
k-means, quantizes to int8, and scans only the nearest nprobe partitions at
query time. Includes shadow table management, insert/delete, KNN integration,
compile flag (SQLITE_VEC_ENABLE_IVF), fuzz targets, and tests. Removes
superseded ivf-benchmarks/ directory.

tests/fuzz/ivf-operations.c

@@ -0,0 +1,121 @@
+#include <stdint.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include "sqlite-vec.h"
+#include "sqlite3.h"
+#include <assert.h>
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  if (size < 6) return 0;
+
+  int rc;
+  sqlite3 *db;
+  sqlite3_stmt *stmtInsert = NULL;
+  sqlite3_stmt *stmtDelete = NULL;
+  sqlite3_stmt *stmtKnn = NULL;
+  sqlite3_stmt *stmtScan = NULL;
+
+  rc = sqlite3_open(":memory:", &db);
+  assert(rc == SQLITE_OK);
+  rc = sqlite3_vec_init(db, NULL, NULL);
+  assert(rc == SQLITE_OK);
+
+  rc = sqlite3_exec(db,
+    "CREATE VIRTUAL TABLE v USING vec0(emb float[4] indexed by ivf(nlist=4, nprobe=4))",
+    NULL, NULL, NULL);
+  if (rc != SQLITE_OK) { sqlite3_close(db); return 0; }
+
+  sqlite3_prepare_v2(db,
+    "INSERT INTO v(rowid, emb) VALUES (?, ?)", -1, &stmtInsert, NULL);
+  sqlite3_prepare_v2(db,
+    "DELETE FROM v WHERE rowid = ?", -1, &stmtDelete, NULL);
+  sqlite3_prepare_v2(db,
+    "SELECT rowid, distance FROM v WHERE emb MATCH ? LIMIT 3",
+    -1, &stmtKnn, NULL);
+  sqlite3_prepare_v2(db,
+    "SELECT rowid FROM v", -1, &stmtScan, NULL);
+
+  if (!stmtInsert || !stmtDelete || !stmtKnn || !stmtScan) goto cleanup;
+
+  size_t i = 0;
+  while (i + 2 <= size) {
+    uint8_t op = data[i++] % 7;
+    uint8_t rowid_byte = data[i++];
+    int64_t rowid = (int64_t)(rowid_byte % 32) + 1;
+
+    switch (op) {
+      case 0: {
+        // INSERT: consume 16 bytes for 4 floats, or use what's left
+        float vec[4] = {0.0f, 0.0f, 0.0f, 0.0f};
+        for (int j = 0; j < 4 && i < size; j++, i++) {
+          vec[j] = (float)((int8_t)data[i]) / 10.0f;
+        }
+        sqlite3_reset(stmtInsert);
+        sqlite3_bind_int64(stmtInsert, 1, rowid);
+        sqlite3_bind_blob(stmtInsert, 2, vec, sizeof(vec), SQLITE_TRANSIENT);
+        sqlite3_step(stmtInsert);
+        break;
+      }
+      case 1: {
+        // DELETE
+        sqlite3_reset(stmtDelete);
+        sqlite3_bind_int64(stmtDelete, 1, rowid);
+        sqlite3_step(stmtDelete);
+        break;
+      }
+      case 2: {
+        // KNN query with a fixed query vector
+        float qvec[4] = {1.0f, 0.0f, 0.0f, 0.0f};
+        sqlite3_reset(stmtKnn);
+        sqlite3_bind_blob(stmtKnn, 1, qvec, sizeof(qvec), SQLITE_STATIC);
+        while (sqlite3_step(stmtKnn) == SQLITE_ROW) {}
+        break;
+      }
+      case 3: {
+        // Full scan
+        sqlite3_reset(stmtScan);
+        while (sqlite3_step(stmtScan) == SQLITE_ROW) {}
+        break;
+      }
+      case 4: {
+        // compute-centroids command
+        sqlite3_exec(db,
+          "INSERT INTO v(rowid) VALUES ('compute-centroids')",
+          NULL, NULL, NULL);
+        break;
+      }
+      case 5: {
+        // clear-centroids command
+        sqlite3_exec(db,
+          "INSERT INTO v(rowid) VALUES ('clear-centroids')",
+          NULL, NULL, NULL);
+        break;
+      }
+      case 6: {
+        // nprobe=N command
+        if (i < size) {
+          uint8_t n = data[i++];
+          int nprobe = (n % 4) + 1;
+          char buf[64];
+          snprintf(buf, sizeof(buf),
+            "INSERT INTO v(rowid) VALUES ('nprobe=%d')", nprobe);
+          sqlite3_exec(db, buf, NULL, NULL, NULL);
+        }
+        break;
+      }
+    }
+  }
+
+  // Final operations — must not crash regardless of prior state
+  sqlite3_exec(db, "SELECT * FROM v", NULL, NULL, NULL);
+
+cleanup:
+  sqlite3_finalize(stmtInsert);
+  sqlite3_finalize(stmtDelete);
+  sqlite3_finalize(stmtKnn);
+  sqlite3_finalize(stmtScan);
+  sqlite3_close(db);
+  return 0;
+}