Prerender gets per-route hasPostponed signal

Patch Changes

• 4e849dd: Add a per-route hasPostponed signal to Prerender.

  @vercel/build-utils exposes a new optional hasPostponed?: boolean field on Prerender / PrerenderOptions. It is a tri-state: true when the route's .meta postponed state is present (React suspended during the build-time prerender), false when the framework prerendered a Prerender route without postponing, and undefined when the framework did not provide the signal.

  @vercel/next populates it for app-router PPR routes (computed from the route's postponed state) and leaves it undefined for pages-router and other non-app-router prerenders. This is an additive, finer-grained signal — it does not change the existing chain / experimentalStreamingLambdaPath behavior — so downstream consumers can distinguish a route that actually postponed from one that has PPR machinery but fully prerendered (e.g. under cacheComponents: true).