Implement useState and useEffect from Scratch

Problem: Implement basic versions of React's useState and useEffect hooks from scratch, without relying on React internals.

Approach: useState: maintain a module-level array of state slots and a cursor index that resets before each render; calling useState(initial) reads/writes the slot at the current cursor position and returns [value, setValue] where setValue triggers a re-render. useEffect: store the dependency array from the previous render alongside the effect callback; on each render, shallow-compare the new dependency array to the stored one and only invoke the effect callback (and cleanup from the previous invocation) if they differ or on first mount.