Partition By

Optional. Reference to a Property used to partition the matched input set for fan-out. When set, the runner reads this property off each matching input, groups inputs sharing the same value, evaluates `rule` and `artifactName` once per group, and emits one Artifact per group. Inside the rule, `key` is bound to the partition value and `inputs` is bound to that partition's members. Inputs that lack the partition property are dropped from the fan-out (no group is created for them). Authors who need a catch-all bucket for "missing partition value" should partition on a derived property that supplies the bucket name.