Derive an operator's configuration from its schema

### Task Summary

An operator is configured by a form, and the form is generated from the descriptor's own annotations. The same annotations are enough to fill it without a human: an enum offers its constants, a column knob takes a column of the type its rule allows, a numeric knob its declared bounds, and a conditional field is filled only where the branch that shows it is taken.

`@SampleColumn` names the column a knob means where a type cannot say it: the opening price among four numbers, the country code among several strings.

This produces one configuration. Turning that configuration into the several a parity run needs is the step after this one. The two halves share no member: this side reaches a value through `buildObject`, `decide` and `valueFor`, and the sweep reaches its own through `rowFills`, `columnFill` and `leafFill`.

Step 21 of 27 in #8325., and the first of that entry's two halves. It reads the `@SampleColumn` annotation, so it needs #8327; two of its tests read that annotation on operators the family steps carry, so they report the first column instead of the named one until those land.