Kotlin — Reactor operator drills (extra practice)

▶ **Extra-practice drills that pair with the Kotlin Reactive course (Chapter 2).** Learn the operators there with prose + visualizations, then drill them here as standalone reps. Real Reactor (`reactor-core`) needs the JVM, so each challenge ships a tiny hand-rolled `Flux<T>` / `Mono<T>` primitive — the candidate implements one *operator* per challenge on top of it: `map`, `filter`, `flatMap`, `concatMap`, `scan`, `zip`, `retryWithBackoff`, `take`/`skip`, `distinct`, `distinctUntilChanged`, `concat`, `merge`, `onError…`, `onErrorResume`, `buffer`, `window`, `sample`, `switchMap`, `takeUntil`. Same mental model as production code. Stdin/stdout via Piston. 15–35 min each.