[ATR-25956] Propagate express terminus summary node through navigation slimming

The express summary node is a single orphaned node in the navigation graph which specifies which accepts were express-accepts.

This wasn't making it out of P1 in the online slimmed navigation summary - due to it being orphaned.

With this update we explicitly propagate it down into all progressively slimmed version of the navigation.

The minor additional payload is introduced at P1 hence this does not violate frozen T0 policy.