chore: scaffold tracks and tooling

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

js/test/helpers.js

@@ -0,0 +1,18 @@
+import { readFileSync } from 'node:fs'
+import { fileURLToPath } from 'node:url'
+import { dirname, join } from 'node:path'
+
+const here = dirname(fileURLToPath(import.meta.url))
+
+/** Load fixtures/<name>.json as { function, cases: [{ name, args, expected }] }. */
+export const loadFixture = (name) =>
+  JSON.parse(readFileSync(join(here, '..', '..', 'fixtures', `${name}.json`), 'utf8'))
+
+/** Deep-freeze for tests (independent of the student's deepFreeze). */
+export const freeze = (x) => {
+  if (x !== null && typeof x === 'object' && !Object.isFrozen(x)) {
+    Object.freeze(x)
+    Object.values(x).forEach(freeze)
+  }
+  return x
+}