App Router dev still fails on raw CommonJS packages from node_modules

[southpolesteve]

Background

PR #665 fixes Pages Router dev for raw CommonJS packages from node_modules by making the direct dev module runner CommonJS-aware. App Router still fails in a separate pipeline.

Reproduction

1. Create an App Router project with app/page.tsx:

import dep from "cjs-node-package";

export default function Page() {
  return <div>{dep.value}</div>;
}

2. Add a CommonJS-only package in node_modules/cjs-node-package/index.js:

module.exports = { value: "from-cjs-package" };

3. Run vinext dev and request /.

Current result

Dev returns 500 and the response includes a Vite/RSC parse error:

A module cannot have multiple default exports

In my repro, that surfaced from the generated/prebundled @vitejs_plugin-rsc_vendor_react-server-dom_server__edge.js path rather than from the package source directly.

Why the Pages Router fix does not apply

• Pages Router uses createDirectRunner() in packages/vinext/src/server/dev-module-runner.ts.
• App Router does not use that runner for request-time module loading.
• App Router goes through @vitejs/plugin-rsc plus vinext's rsc/ssr environments configured in packages/vinext/src/index.ts.
• Those environments currently force noExternal: true (except for explicit externals), so raw node_modules packages are still pushed through Vite/plugin-rsc transforms.

What a fix likely needs

• Add a focused App Router regression test for a raw CommonJS package from node_modules.
• Decide whether these packages should be externalized from the App Router rsc/ssr environments in dev, or transformed into valid ESM/CommonJS-compatible modules before @vitejs/plugin-rsc sees them.
• Keep the existing reasons for noExternal: true in mind: CSS/assets in node_modules, RSC bundling, and React Server condition handling.
• Verify parity against Next.js App Router for CommonJS-only dependencies.

Constraints / notes

• This is a different problem from SSR dev server fails with "module is not defined" for CJS dependencies (void-elements, highlight.js, etc.) #585 after PR fix: support CommonJS node_modules in Pages Router dev #665 lands.
• A direct-runner evaluator fallback is probably not enough here because the failing path is inside the App Router RSC pipeline, not the Pages Router direct-runner path.