[idea] class hierarchies

class syntax is perhaps easy to analyze too, so it might be nice to also show a class inheritance graph. Possibly both can be shown at the same time with different color, or a toggle switch shows one while the other is greyed out, or similar.

The issue names no files, tests, or entry points. Start by locating the existing dependency-graph visualization and class-syntax analysis, then clarify whether inheritance should appear alongside the module graph or through a toggle; done criteria are not specified beyond showing a class inheritance graph.

Written by the indexing model from the issue text.