next/constants shim throws ReferenceError: process is not defined in client bundles

### Summary

Importing anything from `next/constants` in client code crashes production browser bundles with `ReferenceError: process is not defined`.

The shim (`packages/vinext/src/shims/constants.ts`) has a top level bare reference to `process`:

```js
export const CONFIG_FILES = [
"next.config.js",
"next.config.mjs",
"next.config.ts",
// process.features can be undefined on Edge runtime
...(process?.features?.typescript ? ["next.config.mts"] : []),
];
```

Optional chaining does not guard an undeclared identifier, so evaluating the module in a browser throws. Real Next.js has the same expression in its constants module but survives because webpack injects a `process` polyfill into client bundles. Vite does not polyfill `process`.

Since `next/constants` is aliased for every environment (public-shim-map applies to the shared `resolve.alias`), the shim is a legitimate client import. `@sentry/nextjs` for example imports `PHASE_PRODUCTION_BUILD` in code that ends up in the client graph. And because the shim lands in the shared `vinext-*` client manual chunk, the failed evaluation takes down that whole runtime chunk, not just the importer. In our case the visible symptom was Sentry's client SDK never initializing in production.

### Reproduction

Add a `"use client"` page that imports `PHASE_PRODUCTION_BUILD` from `next/constants` to the `app-basic` fixture, build for production, open in a real browser: the page logs `ReferenceError: process is not defined` (we saw 3 page errors from the shared chunk).

### Suggested fix

```js
...(typeof process !== "undefined" && process.features?.typescript ? ["next.config.mts"] : []),
```

Behavior is unchanged in any runtime that has `process`, and the module becomes safe to evaluate in the browser. Verified locally: 0 page errors after the change, same rendered output.

Happy to send a PR. Thanks for vinext, it has been great to run Next apps on Vite!

Start in packages/vinext/src/shims/constants.ts, then inspect the app-basic fixture and its client production-build setup. Reproduce the failure in a real browser and verify the completed change produces no page errors while preserving the rendered output.

Written by the indexing model from the issue text.