Injecting Into the SSR Stream

Nothing should be injected between two React synchronous writes.

According to React's team, chunks should be injected "before React writes": https://github.com/reactwg/react-18/discussions/114#:~:text=Injecting%20Into%20the%20SSR%20Stream
- In practice, chunks indeed cannot be arbitrarily injected, as the app will eventually crash otherwise.
- It isn't clear what "before React writes" means. I interpret it like this: nothing should be injected between two React synchronous writes.
  - So far, my interpreted rule seems to be working.
  - It's also the interpretation of the Apollo GraphQL team: https://github.com/apollographql/apollo-client-nextjs/issues/325#issuecomment-2205375796

Nothing should be injected before the first React write.

This doesn't seem to be documented by the React team.
But, in practice, there seems to (always?) be a hydration mismatch if anything is injected before the first React write.
Reproduction: https://github.com/vikejs/vike/commit/45e4ffea06335ddbcf2826b0113be7f925617daa
Thus, we delay any write to the stream until React writes its first chunk.
Because of Rule 1, all subsequent synchronous React write after the first one also need to be injected first.

Being able to pass a chunk promise to injectToStream() is required for integrating Apollo GraphQL, see: