Move client build ID to a global variable (#72592)
When performing RSC requests, if the incoming data has a different build
ID than the client, we perform an MPA navigation/refresh to load the
updated build and ensure that the client and server in sync.
Currently we store the build ID in the router state, but becauase it's
always in lockstep with the app instance, it's not actually stateful. We
can store it in a global.
The ID gets assigned as a side-effect during app initialization. It
should never change after being set the first time. In practice, because
setAppBuildId is called before hydration starts, it will always get
assigned to the actual build ID before it's ever needed by a navigation.
(If for some reasons it didn't, due to a bug or race condition, then on
navigation the build comparision would fail and trigger an MPA
navigation.)
The logical flow of how the global is assigned is more convoluted than
it should be because we currently decode the Flight stream inside the
React render phase (via a hook), because that's required to propagate
resource hints correctly. Conceptually it would make more sense to
decode the stream before starting hydration and pass the decoded data to
the root component as props; this would also allow us to block hydration
until the id is set. But we'll need to address the hints problem first.
As a follow up, we should probably do the same thing for the App Router
reducer, which is already a global store in practice but is referenced
and passed everywhere as if it were owned by the React tree.
