Built worker fails in workerd: Class extends value async function is not a constructor (wrapWorkerEntrypoint)

## Setup
- vinext 1.0.0-beta.9, App Router under `src/` (`appDir: "src"`), 165 pages
- `vinext build` succeeds; `vinext start` serves correctly (real pages, RSC, rewrites)
- Failure occurs loading the built worker (`dist/server/index.js`) under real workerd via `wrangler dev`

## Error
`Uncaught TypeError: Class extends value async function ... is not a constructor or null` at `wrapWorkerEntrypoint` (minified index.js). Full trace available on request — the heritage expression references minified code around cache-adapter registration (`registerCacheAdapters`).

## Notes
- `vinext start` (Node SSR entry) works, so this looks specific to the worker entry bundle — possibly a minifier emitting an async-function class heritage, which is a spec-level TypeError in every engine.
- No custom minify settings; stock `vinext build` output.
- Related context: same project tracks dev-mode 404s with `src/` layout in #3184.

Reproduce with the reported `vinext build` output and `wrangler dev`, then inspect `dist/server/index.js` around `wrapWorkerEntrypoint` and `registerCacheAdapters`. Compare the worker entry behavior with the working Node SSR entry; done means the built worker loads in real workerd without the class-heritage TypeError.

Written by the indexing model from the issue text.