In the detailed join explain output, change each ndvEstimates entry to:

{
  ndvs: [ { nss: 'shop.suppliers', ndv: 3, ndvSource: 'Sampling' } ],
  joinPredicateSelectivity: 0.3333333333333333
}

• ndvs lists the NDV estimate of every side that was estimated: the smaller collection's side always, and the other side when it is available without sampling (unique index or persisted NDV statistics). It replaces assumedPkSide/ndv/ndvSource, which only show the larger of the two.
• Rename selectivity to joinPredicateSelectivity (= 1 / max(ndv)).

joinPredicateSel in cardinality_estimator.cpp already computes both sides; JoinEdgeSelectivityEstimate needs to keep them and JoinExtraEstimateInfo::serialize needs to output them.