Build: transitive externals resolve to wrong package version

> *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

When `serverExternalPackages` (or equivalent) is configured, transitive dependencies resolve to the wrong version. Test expects `depB: 4.17.21, 4.17.21, 5.0.0` but got `depB: 3.10.1, 4.17.21, 5.0.0` — the first depB instance is wrongly resolved to depA's version.

```
Transitive externals resolution picks wrong package version
```

## Estimated Impact

~1 test failures across the deploy suite.

## Affected Test Suites

- `test/e2e/externals-transitive/externals-transitive.test.ts`

## Recommendation

1. **Reproduce first in vinext's own test suite.** Add a fixture with depA -> depB@4 and root -> depB@3, mark depA as external, and assert each module sees the correct depB version.

2. **Preserve transitive version resolution.** Externalising a package must not collapse nested copies of a transitive dep to a single version.