KIP-260 — add primary join operation for Stream-Stream join (WIP)

Discarded Streams

Proposes a `FirstOrderJoin` (primary join) variant for stream-stream joins that emits at most one output per incoming record by matching only the first found record in the opposite window store. The standard exhaustive window join produces all Cartesian matches, which is semantically wrong for use cases requiring exactly-one join semantics.

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