UML completion transitions: an explicit spelling usable on any base class #699

Why

Reacting when a state finishes its work is the most used form of completion in practice (XState onDone, done.state.* here). UML adds completion of states with a doActivity and runs each completion in its own step. Bare eventless transitions keep their SCXML meaning, so completion needs its own spelling (decided in #692).

Scope

• An explicit completion spelling on transitions, available on StateChart and any subclass, plus the native YAML key, the schema and diagram rendering.
• Completion events are queued and taken one per microstep, so the completions of orthogonal regions run in separate microsteps (PSSM case History_002-B, today unclassified).
• The PSSM converter emits the new spelling, so the cases in its LOWERED map run unlowered.

Acceptance

• LOWERED cases pass without lowering on both engines; History_002-B leaves the xfail map.
• W3C SCXML suite unchanged.

Foundation for join, the doActivity completion rule (K2) and deferral recall order.

Refs #692