Zigroutines: new concurrency lib for Zig

zigroutines zigroutines is a library that brings stackful M:N concurrency to Zig, inspired by Go’s task model, channels, and select, but with a philosophy that fits Zig perfectly: nothing global, nothing hidden, pay only for what you enable. You construct a Runtime with your chosen scheduler (FIFO, work‑stealing, priority, or even 1:1 OS threads), I/O backend (poll, IOCP, io_uring, or none), and optional metrics. Every task gets a fixed 2 KiB stack (no silent growth, no per‑task configuration)...