Start prerendering Suspense retries immediately (#30934)

When a component suspends and is replaced by a fallback, we should start
prerendering the fallback immediately, even before any new data is
received. During the retry, we can enter prerender mode directly if
we're sure that no new data was received since we last attempted to
render the boundary.

To do this, when completing the fallback, we leave behind a pending
retry lane on the Suspense boundary. Previously we only did this once a
promise resolved, but by assigning a lane during the complete phase, we
will know that there's speculative work to be done.

Then, upon committing the fallback, we mark the retry lane as suspended
— but only if nothing was pinged or updated in the meantime. That allows
us to immediately enter prerender mode (i.e. render without skipping any
siblings) when performing the retry.