refactor: use is_initial flag and hide implicit initial transitions

- Add is_initial field to DiagramState IR, extracted from state metadata
- Use is_initial to determine which state gets the black-dot initial
  arrow instead of relying on document order heuristics
- Skip rendering implicit initial transitions from compound/parallel
  states to their initial child — these are already represented by the
  black-dot initial node inside the cluster
- Skip initial arrows for parallel area children (all are auto-initial)
- Regenerate affected documentation images

Author: fgmacedo