feat: diagram generation for compound, parallel, and history states

- Render initial pseudo-state (black dot → initial child) inside all
  compound subgraphs, not just root level
- Add history state rendering as UML circles labeled "H" / "H*"
- Annotate parallel state subgraph labels with ☷ indicator
- Support multi-target transitions (one edge per target)
- Extract _add_transitions() helper to reduce _graph_states() complexity
- Remove stale TODO comment
- Fix NestedStateFactory to propagate kwargs (e.g. parallel=True) from
  State.Parallel/State.Compound base classes to subclass-created States
- Export HistoryState from statemachine.__init__
- Register event label "None" bug as release blocker in PLAN

Author: fgmacedo