Coverage Draft speed training

30/30 correct. PB 3.337s held; session best 3.620s. Reuse: A–F fixed; BDF costs 11. Read G cost first: >=5 => BDF immediately. Otherwise use the cached cheapest A–F pair covering G’s complement; take G+pair only if cheaper, or tied and lex-smaller. No A–F rescan. Bottleneck: runtime variance.