Set Transformation

Aggregate transformation operating on the matched SET of inputs. Without `partitionBy`, the runner evaluates `rule` and `artifactName` once with `inputs` (plural) bound to the entire matched set, producing 1 artifact (per output format). With `partitionBy`, the runner splits the matched set into disjoint subsets keyed by the partition property, evaluates `rule` and `artifactName` once per subset with `inputs` bound to that subset's members and `key` bound to the partition value, producing M artifacts (one per distinct partition key). Empty match set is not an error — the rule still evaluates with `inputs` bound to an empty list, and one Artifact is emitted. This differs from InstanceTransformation, where zero matches means zero artifacts.