[Fizz] Track boundaries in future rows as postponed (#33329)

Follow up to #33321.

We can mark boundaries that were blocked in the prerender as postponed
but without anything to replayed inside them. That way they're not
emitted in the prerender but is unblocked when replayed.

Technically this does some unnecessary replaying of the path to the
otherwise already completed boundary but it simplifies our model by just
marking the boundary as needing replaying.