Fix dev error page bootstrapping for App Router
The in-memory __next_error__ HTML shell now correctly bootstraps the
turbopack runtime by loading all rootMainFiles as <script async> tags
(mirroring React's renderToReadableStream behavior) instead of <script
defer> in the head. The last rootMainFile is the turbopack runtime
bootstrap which processes all buffered chunk registrations and executes
the app entry point.
On the client side, when the __next_error__ shell is detected, we skip
the full App Router machinery (which would crash without valid RSC
payload/router state) and instead replay any embedded error via
handleClientError so the dev overlay shows it immediately.
Also improved App Router route detection in mixed projects by using
getOriginalAppPaths() for auto-detection instead of requiring explicit
request tagging.
Co-Authored-By: Claude <noreply@anthropic.com>
