UML junction pseudostate (and segmented compound-transition execution) #715

Why

Static conditional branches are already expressible with guarded transitions; the pseudostate matters for UML models and for six PSSM cases. It needs the engine to run a compound transition segment by segment (effects interleaved with exits and entries), which today's microstep does not do. Junction carries that executor because its cases pin the order with the smallest review; entry/exit points, choice and join reuse it.

Refs #692