Support parent-child state machine communication in Python API

[fgmacedo]

Summary

When using <invoke> via SCXML, parent and child state machines can exchange events using <send target="#_parent"> (child → parent) and <send target="#_<invokeid>"> (parent → child). This communication protocol is not yet available when defining state machines in Python.

Current state

Direction	SCXML	Python (IInvoke)	Python (callable / StateChart)
Parent → Child	<send target="#_<invokeid>">	No	No
Child → Parent	<send target="#_parent">	ctx.send()	No

• IInvoke handlers can send events to the parent via ctx.send(), but plain callables, naming convention methods (on_invoke_<state>), and child StateChart classes have no way to communicate back.
• There is no public API for the parent to send events to a running child machine.

Gaps

1. StateChartInvoker lacks on_event() — InvokeManager.send_to_child() requires the handler to implement on_event(event, **data), but StateChartInvoker (used for State(invoke=ChildMachine)) doesn't have it.
2. _invoke_session not set for Python children — StateChartInvoker.run() instantiates the child without passing _invoke_session, so the child has no reference to the parent. Only SCXMLInvoker sets this up.
3. No public API on StateChart — send_to_child() exists on InvokeManager but is internal (sm._engine._invoke_manager). There's no user-facing method.
4. Invoke ID not exposed — users have no way to obtain the invokeid of a running invocation (SCXML uses idlocation= for this).

Possible approach

• Add on_event() to StateChartInvoker that forwards to self._child.send().
• Pass _invoke_session when instantiating Python child machines so they can send events to the parent.
• Expose a public method on StateChart for parent → child communication (e.g., sm.send_to_invoke(invokeid, event, **data)).
• Consider an autoforward option for State(invoke=..., autoforward=True).