UDFs via a columnar JVM upcall

Applies to: user ScalarFunctions the native engine can't implement itself

A user ScalarFunction the native engine can't implement itself runs inside the island instead of falling the whole query back to Flink: the argument columns are packed into one batch, exported over the C Data Interface, evaluated by the real function on the JVM, and the result column imported back — one JNI crossing per batch, never per row. The design is modelled on Comet's JvmScalarUdfExpr.

Because Flink's own code computes the values, the result is byte-identical to Flink by construction — there is no reimplementation to diverge.

Functions are serialized into the operator and registered per-task at open(), so this survives distributed execution, where the UDF instance must be reconstructed on each task's JVM rather than shared from the planner.