fix(module): ask Node whether a specifier is a builtin

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>

server/scripts/checkImports.js

@@ -28,19 +28,23 @@
 // because a test that reaches into core's tree is a test that passes on this
 // machine and nowhere else.
 //
-// Run over the SERVER half. The client half's equivalent is its Vite build:
-// the four shared dependencies are `external`, and anything else that stays a
-// bare import in the emitted chunk is unresolvable in the browser.
+// Run over the SERVER half. The client half's equivalents are its Vite build,
+// which fails if a shared dependency resolves into node_modules, and
+// client/scripts/checkExternals.js, which asks the built chunk whether any bare
+// specifier survived.
 
 const fs = require('fs')
 const path = require('path')
-const { builtinModules } = require('module')
+// Node's own answer, not a list reconstructed from `builtinModules`. That list
+// omits `test` on Node 20 and includes it on Node 24, so a suite that requires
+// `node:test` passed locally and failed in CI on the very first run — reported
+// as the module boundary being broken, which it was not. `isBuiltin` is the
+// authoritative check and handles the `node:` prefix itself.
+const { isBuiltin } = require('module')
 
 const MODULE_ROOT = path.resolve(__dirname, '..', '..')
 const SERVER_ROOT = path.join(MODULE_ROOT, 'server')
 
-const BUILTINS = new Set([...builtinModules, ...builtinModules.map((m) => `node:${m}`)])
-
 // Dependencies this half is allowed to resolve for itself. Empty, and that is
 // the design: everything the server half needs comes from `ctx` (§2.3). A new
 // entry here is a real decision — it becomes a package an operator's install
@@ -148,7 +152,10 @@ function scan(root, moduleRoot = MODULE_ROOT, { shipped = isShipped, dev = devDe
           ? specifier.split('/').slice(0, 2).join('/')
           : specifier.split('/')[0]
         const allowed = ALLOWED_PACKAGES.has(pkg) || (!shipped(file) && dev.has(pkg))
-        if (!BUILTINS.has(specifier) && !BUILTINS.has(pkg) && !allowed) {
+        // The `node:` prefix can only ever name a builtin, so it never reaches
+        // node_modules and is safe whatever this Node version enumerates.
+        const builtin = isBuiltin(specifier) || specifier.startsWith('node:')
+        if (!builtin && !allowed) {
           violations.push({ file, specifier, why: 'undeclared bare specifier — should this come from ctx?' })
         }
       }

server/test/checkImports.test.js

@@ -81,6 +81,19 @@ test('allows node builtins anywhere, with or without the node: prefix', () => {
   )
 })
 
+test('allows node:test, which older Node versions omit from builtinModules', () => {
+  // The first CI run failed on exactly this and on nothing else: `builtinModules`
+  // omits `test` on Node 20 and includes it on Node 24, so every test file in
+  // this suite was reported as breaking the module boundary. The check asks
+  // Node (`isBuiltin`) rather than rebuilding the list, and treats the `node:`
+  // prefix as sufficient on its own — a prefixed specifier can never resolve to
+  // a package, whatever the running version enumerates.
+  assert.deepStrictEqual(
+    scanFixture({ 'a.js': `require('node:test'); require('node:test/reporters')` }),
+    [],
+  )
+})
+
 test('catches ESM and dynamic forms, not only require()', () => {
   const found = scanFixture({
     'a.js': [`import db from '../../core/db.js'`, `const x = await import('../../core/other.js')`].join('\n'),