Bots get Suspense fallbacks instead of full HTML since 1.170.37 #8687

Which project does this relate to?

Router

Describe the bug

Since 1.170.37 (#8204), bots get Suspense fallbacks instead of the full HTML.

renderRouterToStream creates the response stream before it awaits stream.allReady. That stream (makeMergeStream in transformStreamWithRouter.ts) has the default highWaterMark: 1, so it pulls from React as soon as it is created. React sends the shell early and streams the rest.

Reproduces on Node and on Cloudflare Workers.

Complete minimal reproducer

https://github.com/GregoryCollett/tanstack-start-bot-repro

Steps to Reproduce the Bug

1. npm install && npm run build && npm start
2. curl -A Googlebot http://localhost:3000/

Expected behavior

The response contains <p>slow content</p>, as it does on 1.170.36. On 1.170.42 it contains <p>loading</p> in a pending <!--$?--> boundary.

Platform

• Router / Start Version: react-router 1.170.42, react-start 1.168.61
• OS: macOS
• Browser: n/a (curl with a bot user agent)
• Bundler: vite
• Bundler Version: 8.3.4

Additional context

Passing highWaterMark: 0 fixes it for us, and browsers still stream:

const stream = new ReadableStream<Uint8Array>(
  { start, pull, cancel },
  { highWaterMark: 0 },
)