refactor: reduce compound state padding and fix integrations doctest

- Add margin="4" to compound/parallel subgraphs to reduce default
  Graphviz cluster padding (was 8pt default)
- Place anchor nodes directly on parent graph when no atomic states
  exist at that level, avoiding empty cluster whitespace
- Remove graph_dpi config (use Graphviz default)
- Fix integrations.md doctest: register CampaignMachine in registry
  and skip Django autodiscovery to prevent ImproperlyConfigured error
- Merge doctest blocks so context carries across examples
- Regenerate all documentation images

Author: fgmacedo