refactor: improve compound state edge routing with bidirectional anchors

- Compound states with both incoming and outgoing transitions get
  separate _anchor_in/_anchor_out nodes for cleaner edge routing
- Move anchor nodes into the atomic subgraph so they share the same
  rank region as real states, avoiding blank space in clusters
- Place inner initial dots in the atomic cluster for shorter arrows
- Extract _render_initial_arrow helper to reduce _render_states complexity
- Regenerate affected documentation images

Author: fgmacedo