Build failure: Babel flowtype page compilation not supported

> *This issue was created by an agent analysing CI failures from the [Next.js Deploy Suite](https://github.com/cloudflare/vinext/actions/runs/26282973805) (vinext `main` vs Next.js `v16.2.6`, 2026-05-22).*

## Problem

Pages using flowtype syntax (compiled via Babel + `@babel/preset-flow`) fail the vinext build with `Custom deploy script failed: undefined (1)`. The Vite/Rolldown pipeline does not run user Babel configuration for these files.

```
Babel + preset-flow page fails to build
```

## Estimated Impact

~1 test failures across the deploy suite.

## Affected Test Suites

- `test/e2e/babel/index.test.ts`

## Recommendation

1. **Reproduce first in vinext's own test suite.** Add a page using flow syntax with a `.babelrc` enabling `@babel/preset-flow`, confirm the build fails.

2. **Decide on Babel support.** Either add an opt-in Babel-compatibility loader for files matched by user `.babelrc`, or document that Babel is not portable and convert the fixture to TS.