feat(js): m01 util and m02 model with tests, fixtures, purity audit

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

js/test/helpers.js

@@ -8,11 +8,12 @@ const here = dirname(fileURLToPath(import.meta.url))
 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)) {
+/** Deep-freeze for tests (independent of the student's deepFreeze). Recurses into children even of already-frozen objects. */
+export const freeze = (x, seen = new WeakSet()) => {
+  if (x !== null && typeof x === 'object' && !seen.has(x)) {
+    seen.add(x)
     Object.freeze(x)
-    Object.values(x).forEach(freeze)
+    Object.values(x).forEach((v) => freeze(v, seen))
   }
   return x
 }