The walk's predicates partition the artifact's key space, so a merge that
ends with fewer rows than the artifact declares does not mean the artifact
was smaller than it said — it means a predicate filtered rows out, and
the catalog is quietly partial while reporting complete.
Equality rather than a lower bound: RowsAffected counts an upsert that
changes nothing, and a row already merged locally is counted again here.
One reachable case, so this is not merely a tripwire. A row whose mbid is
empty is excluded by `mbid > ?` in both encodings, so an artifact
carrying one imports as a success with a row missing — which is the shape
#258 had, one cause over. The test covers exactly that artifact.
Refs #258
yonlu
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