Export the hierarchy and graph visualizations as Python

### Task Summary

Seven visualizations implement `StandaloneCodeGenerator`: the ones that draw a structure rather than a coordinate system, tree plot, network graph, sankey, hierarchy chart, icicle chart and dendrogram, plus the word cloud.

A visualization emits a figure rather than a frame, so it is compared as one, number by number after both paths have drawn, which catches a difference in the data behind the picture without failing over a layout detail that carries no meaning. The word cloud renders an image instead, so it is compared as the HTML that carries it.

Network Graph and Word Cloud are verified here rather than withheld. Both had been flagged for drawing a different picture on every run, which stopped being true once their placement was seeded, and lifting those two rows showed three things the reason had been hiding. Network Graph never called `manipulateTable`, the method it already had for dropping the rows that name no edge, so a null reached `add_node`, which refuses one outright. Word Cloud counted its words out of whatever column it was given, and the filter that finds them uses pandas' `.str`, which refuses anything but text, so the operator now states that. And the Tree Plot lays itself out without igraph, which is GPL v2 and Category X under the ASF third-party licence policy.

Step 13 of 27 in #8325. It needs #8327 for the trait. The Tree Plot's dependency is filed separately as #7969 and closes with this work.