Export the charts drawn over a field or a sequence

### Task Summary

Ten visualization operators still have no standalone form, so a workflow ending in one of them exports a script with a commented placeholder where the chart should be. What they have in common is the question they answer: how a value varies across a domain, rather than where a single row sits on an axis. Filled Area, Contour, Dumbbell, Time Series, Quiver, Choropleth Map, 3D Scatter, Volcano, Gantt and Carpet.

Each implements `generateStandaloneCode`, emitting the plotly its executor emits and writing the page to the file the translator names for it, so a plan holding two charts writes two files rather than one overwriting the other.

A chart that cannot be drawn writes the reason to that page rather than printing it. The engine's operator yields a page either way, so an export that printed instead would leave the run with no file where the workflow had one, which reads as a crash.

Step 17 of 27 in #8325.