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 filter flags (--filter,
--included-by, --references, etc.). Use --include-headers for detailed
visualization with section subgraphs. Combining -k with filter flags
intersects the two — only explicit keys that match the filter are exported.
