_cluster_reassign falls back to hash grouping only where sorting raises, keeping
sortable ids byte-identical to 4.65.0 (measured 0/1200 differing; an
unconditional switch changed 724/1200). The shot arm clusters on
(provider, game_id), so pooling providers cannot fuse two matches that share a
game_id value and corrupt the cluster-exchangeable null.

No retrain. C4 count unchanged (32).