With --experimental_cluster_shard_by_slot, Shard() mapped keys with KeySlot(key) % shard_num.
CRC16 slots of structured key sets can share a residue modulo the shard count, piling keys
onto a single shard thread.

Map the slot through Fibonacci hashing and a multiply-shift range reduction instead. This breaks
slot-number patterns while keeping contiguous slot ranges evenly balanced, and avoids the
division. Keys within one slot (including hash tags) still share a shard.