Export graph structure in various formats for visualization and analysis.

Available formats:
- dot: Graphviz DOT format for graph visualization

Filter scope with -k/--key (repeatable) or with the structural set selector
(--in / --in-any / --not-in / --max-depth). Use --include-headers for detailed
visualization with section subgraphs. Combining -k with the selector intersects
the two — only explicit keys that match the selector are exported.
