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-quantize.c

@@ -0,0 +1,54 @@
+#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>
+
+/* These are SQLITE_VEC_TEST wrappers defined in sqlite-vec-rescore.c */
+extern void _test_rescore_quantize_float_to_bit(const float *src, uint8_t *dst, size_t dim);
+extern void _test_rescore_quantize_float_to_int8(const float *src, int8_t *dst, size_t dim);
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  /* Need at least 4 bytes for one float */
+  if (size < 4) return 0;
+
+  /* Use the input as an array of floats. Dimensions must be a multiple of 8
+   * for the bit quantizer. */
+  size_t num_floats = size / sizeof(float);
+  if (num_floats == 0) return 0;
+
+  /* Round down to multiple of 8 for bit quantizer compatibility */
+  size_t dim = (num_floats / 8) * 8;
+  if (dim == 0) dim = 8;
+  if (dim > num_floats) return 0;
+
+  const float *src = (const float *)data;
+
+  /* Allocate output buffers */
+  size_t bit_bytes = dim / 8;
+  uint8_t *bit_dst = (uint8_t *)malloc(bit_bytes);
+  int8_t *int8_dst = (int8_t *)malloc(dim);
+  if (!bit_dst || !int8_dst) {
+    free(bit_dst);
+    free(int8_dst);
+    return 0;
+  }
+
+  /* Test bit quantization */
+  _test_rescore_quantize_float_to_bit(src, bit_dst, dim);
+
+  /* Test int8 quantization */
+  _test_rescore_quantize_float_to_int8(src, int8_dst, dim);
+
+  /* Verify int8 output is in range */
+  for (size_t i = 0; i < dim; i++) {
+    assert(int8_dst[i] >= -128 && int8_dst[i] <= 127);
+  }
+
+  free(bit_dst);
+  free(int8_dst);
+  return 0;
+}