Partition By

Iterates `source`, evaluates `partitionKey` per element with `loopVar` bound, and groups elements by their key value. Returns a list of Partition embeddeds, each with a `key` and `members`. Authors typically iterate the result with a ForEach to render per-group sub-structure. NOTE: This is the EXPRESSION-level partition (groups inside a rule). The transformation-level fan-out partition is the SetTransformation.partitionBy property — same operation, different scale. Pick the right one for your use: partition inside a single artifact (this) vs. partition into separate artifacts (the SetTransformation property).