mobxjs/mobx ? reverse-engineered prompt

Build me a simple, scalable state management library for JavaScript and TypeScript that works with React too.

I want to be able to define some app state, update it with normal JavaScript assignments and methods, and have anything that reads that state update automatically when it changes. Please include the core library, a small React friendly way to observe changes, and a clear example showing a counter or timer working end to end. Make it feel lightweight and easy to use, with minimal boilerplate, and keep the API straightforward enough that state can live outside the UI if needed.

Please also add good tests and a basic docs site or examples so it is easy to understand how to use it. If you need current docs or best practices for any modern setup, look them up online first.