Precision Run speed training

21/21 correct; PB 3.824s→3.602s. Reuse: preserve issued order; reverse and digit-sum directly; hex by 256-bucket (0/1/2/3) plus cached low-byte nibbles, emit typed 12-item array immediately with no regroup/check. Faster reps had fewer costly hex conversions; slower reps were hex/latency-heavy. Bottleneck: runtime variance + hex density.