Sweep a configuration into the variants a parity run needs

### Task Summary

One configuration exercises one path through an operator. A parity run wants more than that: every value an enum offers, the optional knobs a default leaves empty, and a text knob carrying a value that breaks a Python literal if it was spliced rather than escaped.

This adds the sweep on top of the generator that derives the configuration. Four entry points return variants: one for an operator configured from its annotations, and three for a hand-written fixture, which needs them more, since the configuration it pins is otherwise the only one its operator ever runs.

The sweep moves one knob at a time rather than crossing them, so a failure names the knob that caused it. The exceptions are the two variants that move every knob of a kind at once, where bisecting by hand costs less than a run per field: all optional knobs filled together, and all free-text knobs carrying the hostile value together, numbered so no two knobs carry the same text. Two knobs sharing one value is not a smaller version of the same test: where both name an output column, it writes the same column twice and the run fails for a reason that has nothing to do with escaping.

Step 22 of 27 in #8325., and the second of the seventeenth entry's two halves. The two are one Scala object, so this one does not compile without the members the first defines.