Run an operator through the engine and keep what it wrote

### Task Summary

An operator's answer cannot be compared against anything until there is a way to get one. This step adds `OpExecHarness`: it builds the physical operator a descriptor describes, feeds it the rows of a JSONL file per input port, and writes what each output port produced back out, schema in a sidecar because JSONL carries values alone and cannot say a column is an integer rather than a number.

The tests that need a Python interpreter are tagged and split into a job that provisions one, so the job that does not stays as fast as it was.

Step 3 of 27 in #8325. It stands on its own: it touches no code from the export itself.