Identity expressions admitted as passthroughs

Applies to: the ROWTIME materializer Calc, PROCTIME() materialization

Flink's rowtime materializer Calc (Reinterpret(CAST(rt)) — both value-identity) and PROCTIME() materialization were rejected by the encoder, which kept whole event-time and proctime pipelines on the host even though neither expression does any real computation.

Encoding them as passthrough columns / a per-batch literal keeps the island whole at zero compute cost. The proctime value itself is never observed in output — it's an arrival-order signal that gets projected away — so admitting it as a passthrough carries no correctness risk despite PROCTIME() being non-deterministic in general.