Batch / aggregate config from [batch]. Present only on aggregate nodes.

When set, the node processes messages in batches rather than one at a time: its pure_core receives the list of payloads in the batch (not a single payload) and reduces them; its effect_shell emits a single aggregate result (or splits / drops, like any node). The batch flushes when it reaches size messages or after timeout_ms of idle — whichever comes first, so a partial batch is never stranded.