ReactiveX/rxjs ? reverse-engineered prompt

Build me the next generation of RxJS for JavaScript and TypeScript.

I want a reactive programming library that makes it easy to work with streams of values, events, and async work using Observables. It should support a native web platform Observable when the runtime has one, and use a compatible fallback when it does not. I want operators and helpers to be used in a clean, modern way with symbol based extensions, plus a ColdObservable type for cases where each subscription should create its own producer. Cancellation should work with AbortSignal, and the published code should be ESM only.

Please set up the core library, the observable polyfill piece, and a solid test setup with marble style and virtual time testing. Add enough docs and examples so someone can see how to create a stream, transform it, and subscribe to it. If anything is unclear, use the current docs and the latest platform behavior as a guide.