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

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

js/src/todo.js

@@ -0,0 +1,4 @@
+// Placeholder body for unimplemented stubs: replace each `todo('mNN')` call with real code.
+export const todo = (module) => {
+  throw new Error(`TODO: implement in ${module}`)
+}