docs: explain runtime environment App Shells

[DavidIlie]

What?

Document how server runtime environment variables interact with Cache Components and Partial Prefetching.

The guide now shows how to place io() before an environment-dependent branch, when to use connection() instead, what runtime prefetches contain, and why a streamed notFound() remains 200 with noindex. The self-hosting and Partial Prefetching pages cross-link the guidance.

Why?

Reading process.env is not itself a Request-time API. A synchronous branch can run during next build, so conditional UI, notFound(), or redirect() can become part of the App Shell. Starting the same Docker image with a different value does not regenerate that prerendered result.

This can be especially confusing when metadata is dynamic but the page component is not: a navigation may receive runtime metadata while the body still comes from a build-time error shell.

How?

• Explain that the dynamic marker must appear before every environment-dependent branch.
• Use io() for synchronous uncached runtime I/O and connection() when evaluation must wait for an actual navigation.
• Clarify that, in the uncached example, both leave a hole during runtime prefetch and evaluate during the full dynamic render.
• Explain the streamed status-code tradeoff and point hard-404 requirements to a lightweight check in Proxy.
• Document the parent PR's [status: 404] build annotation as an informational diagnostic.

This PR is stacked on #97125, which adds the build-output diagnostic it documents.

Verification

• Targeted Prettier and ESLint
• git diff --check

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