The first CI run failed on `node:test`, in every test file, reported as the
module boundary being broken. It was not: `builtinModules` omits `test` on
Node 20 (CI) and includes it on Node 24 (local), so a list rebuilt from it
disagrees with itself across versions.
Use `isBuiltin`, which is Node's own answer, and treat the `node:` prefix as
sufficient on its own -- a prefixed specifier can never resolve to a package,
whatever the running version enumerates. Test covers both forms.
Also corrects this file's header: the client half's guard is no longer
`external` (it never worked), it is the Vite build's resolution-time check plus
checkExternals.js on the built chunk.
Co-Authored-By: Claude <noreply@anthropic.com>
wtclaude
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