Add Fiber::Scheduler#io_splice + IO-uring backing for IO.copy_stream.

[ioquatix]

Summary:
Build an async-aware, zero-copy data-transfer path in Ruby by exposing Linux’s splice(2) through the Fiber Scheduler, and wiring it up so IO.copy_stream can take advantage of io_uring when available.

Why it matters:
Large file copies and proxying workloads become dramatically faster and cheaper because the data never touches user space. This gives Ruby a modern, high-performance primitive for bulk I/O.