Function std::http_serve_reactor

int http_serve_reactor(unsigned short port, HttpHandler handler, int numThreads)

Same contract as http_serve()/http_serve_threaded(), but each of 'numThreads' OS threads runs a non-blocking std::Reactor/TaskScheduler event loop (see std/sched/reactor.h) instead of blocking one connection at a time — many concurrent connections per thread rather than one, the same shape async runtimes elsewhere use for higher throughput under concurrent load. Requires a reactor backend (reactor_kqueue.sc/reactor_epoll.sc/reactor_win32.sc) to already be included by the caller, same convention as io_nb.h's own backend selection. Returns -1 immediately on setup failure; does not return otherwise.