KIP-857 — Streaming recursion in Kafka Streams

Discussion Streams

Adds a `KStream.recursively(UnaryOperator<KStream> op)` DSL operator that pipes a stream's output back into an earlier stage of the same topology, enabling iterative graph traversal and recursive algorithms natively. Recursive patterns currently require separate loopback topics and a second `KafkaStreams` instance, incurring extra serialization, latency, and operational complexity.

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