nightly

fix(engine): keep interposed step routing live across a loop entry

A loop entry only replaces its own downstream closure. A routing step
outside that closure (a parallel branch, or anything upstream) keeps
its existing instance as current, even after the issue's loop counter
moves past its ordinal. Staleness was read from the ordinal alone, so
those steps were wrongly treated as superseded: their routing recorded
but nothing acted on it, leaving an interposed gate stuck pending and
the issue never completing.

Staleness now also requires a later instance of the same step to
actually exist, matching what "superseded" means.