Add rescore index for ANN queries

Add rescore index type: stores full-precision float vectors in a rowid-keyed
shadow table, quantizes to int8 for fast initial scan, then rescores top
candidates with original vectors. Includes config parser, shadow table
management, insert/delete support, KNN integration, compile flag
(SQLITE_VEC_ENABLE_RESCORE), fuzz targets, and tests.

tests/fuzz/rescore-create.c

@@ -0,0 +1,36 @@
+#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) {
+  int rc = SQLITE_OK;
+  sqlite3 *db;
+  sqlite3_stmt *stmt;
+
+  rc = sqlite3_open(":memory:", &db);
+  assert(rc == SQLITE_OK);
+  rc = sqlite3_vec_init(db, NULL, NULL);
+  assert(rc == SQLITE_OK);
+
+  sqlite3_str *s = sqlite3_str_new(NULL);
+  assert(s);
+  sqlite3_str_appendall(s, "CREATE VIRTUAL TABLE v USING vec0(emb float[128] indexed by rescore(");
+  sqlite3_str_appendf(s, "%.*s", (int)size, data);
+  sqlite3_str_appendall(s, "))");
+  const char *zSql = sqlite3_str_finish(s);
+  assert(zSql);
+
+  rc = sqlite3_prepare_v2(db, zSql, -1, &stmt, NULL);
+  sqlite3_free((void *)zSql);
+  if (rc == SQLITE_OK) {
+    sqlite3_step(stmt);
+  }
+  sqlite3_finalize(stmt);
+  sqlite3_close(db);
+  return 0;
+}