Initialize rescore distance variable to FLT_MAX

The `dist` variable in rescore KNN quantized distance computation was
uninitialized. If the switch on quantizer_type or distance_metric
didn't match any case, the uninitialized value would propagate into
the top-k heap, potentially returning garbage results.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: asg017

sqlite-vec-rescore.c

@@ -465,7 +465,7 @@ static int rescore_knn(vec0_vtab *p, vec0_cursor *pCur,
     for (int j = 0; j < p->chunk_size; j++) {
       if (!bitmap_get(b, j))
         continue;
-      f32 dist;
+      f32 dist = FLT_MAX;
       switch (vector_column->rescore.quantizer_type) {
       case VEC0_RESCORE_QUANTIZER_BIT: {
         const u8 *base_j = ((u8 *)baseVectors) + (j * (qdim / CHAR_BIT));