Identity expressions admitted as passthroughs

Applies to: the ROWTIME materializer Calc

Flink's rowtime materializer Calc (Reinterpret(CAST(rt)) with a widening timestamp cast) changes only the declared type. Encoding it as an identity projection reuses the timestamp column and keeps event-time pipelines native without copying values.

PROCTIME() materialization also remains native. It now uses a volatile execution-time clock, sampled during each batch evaluation, so projected values advance with execution. Proctime ordering in deduplication and OVER still follows arrival order; see Temporal functions.