prevent assets fall through

[nichtsam]

Currently, requests to /assets/* that don’t match an existing asset fall through to the Remix handler.
I think we can assume that these requests are always for assets and never for pages, so it’s safe to respond with 404 directly.

This avoids unnecessary page routing and can significantly reduce overhead in rare cases.
For example, when opening DevTools on a production site, the browser automatically requests all sourcemap files, which don’t exist in production. Without this change, each of these requests triggers the root loader, including all authentication logic and database access.

Test Plan

Checklist

• Tests updated
• Docs updated

Screenshots

[kentcdodds]

Thanks 👍