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>