Server actions fail with [vite-rsc] invalid server reference on cold-start in dev

Repro: App Router project, 'use client' component statically imports a 'use server' action, action invoked before browser has fetched the action file (e.g. dev-server restart while browser kept stale modules, or any race where the action call lands first).

Cause: @vitejs/plugin-rsc populates serverReferenceMetaMap lazily as files are transformed. Validation runs before the lazy import that would trigger the transform, so cold validation throws [vite-rsc] invalid server reference '/path/to/action.ts' and the action never runs. Tracked upstream as [#641](https://github.com/vitejs/vite-plugin-react/issues/641).

Workaround (in user's vite.config.ts):

```
environments: { rsc: { dev: { warmup: ['./lib/actions/**/*.ts'] } } }
```
Suggested fix: vinext could auto-inject dev.warmup for any file under app/ (and configurable action dirs) containing 'use server'.
Since vinext already scans for App Router files, adding 'use server' detection during that scan is cheap. Users wouldn't need any config.

Start with vinext's existing App Router file scan and the dev configuration represented by vite.config.ts. Reproduce the cold-start race with a statically imported use server action, then trace where server actions are detected and warmup entries are assembled. Done means eligible action files are automatically warmed in dev, including configurable action directories, without user configuration.

Written by the indexing model from the issue text.