Submachine states: reuse a state machine class as a state #704

Why

Composing machines out of reusable machines has the strongest demand found: statelyai/xstate#221 and #428, dotnet-state-machine/stateless#232, spring-statemachine#801, and here #416, #614 and #616. Today invoke= runs a child machine with its own queue, which is not the same as embedding its states. Previously out of scope for #692 because PSSM has no submachine tests; reopened by demand, so semantics come from the UML specification text.

Scope

• Embedding a StateChart subclass as a compound state of another machine, with id prefixing and callback resolution.
• Connection points (entry/exit points) are a later step.

Acceptance

Tests on both engines: the embedded machine's states, transitions and callbacks behave as if declared inline; two embeddings of the same class do not share state.

Depends on #660 (per-class copies of the state graph). Refs #692