buckets in the last split; 0 if Design B declined.
Test-only count of buckets Design B's split produced; 0 means it did not run.
Design B and the serial LSD loop answer identically by construction, so this is the only way to tell which one ran -- and therefore the only way any test of the split, the bucket cap or the balance test can be non-vacuous. Zero is informative rather than missing: it is exactly what a declined split looks like, which is the normal outcome on a low-cardinality key.