fix(js): keep test/helpers.js free of src imports

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

js/test/purity.test.js

@@ -73,3 +73,7 @@ jsFiles(srcDir)
   .forEach((f) =>
     test(`purity: ${f.slice(srcDir.length + 1)} has no banned constructs`, () =>
       assert.deepEqual(violations(readFileSync(f, 'utf8')), [])))
+
+// helpers.js is loaded by every test file; importing src/ there would make all tests fail at load time against the stubs.
+test('test/helpers.js imports nothing from src/', () =>
+  assert.doesNotMatch(readFileSync(join(dirname(fileURLToPath(import.meta.url)), 'helpers.js'), 'utf8'), /from\s+['"][^'"]*src\//))