Reactive & Flow API

Flow API is the JDK's four-interface Reactive Streams contract where the subscriber calls `request(n)` to pull demand so a fast publisher never floods a slow consumer.

Flow, Reactor and RxJava speak the same Reactive Streams contract; reactive is for backpressured streams, CompletableFuture is for a single async result, and virtual threads let simple blocking code scale so you only go reactive when you truly need streaming backpressure.