Distinct-cold-key count above which a prefetch routes to the sorted, sharded
batch read instead of per-key parallel point-gets. Shared by the account and
storage prefetch gates here and by the merkle trie-node prefetch gate in
ethrex-blockchain, so tuning it moves all three together. ~16384 cold
accesses is ~34M gas of cold reads: above ordinary cold blocks, below the
large-state blocks these paths target. Tunable.