Richer process APIs using pidfds (Fiber::Scheduler#process_open).

[ioquatix]

Summary:
Introduce pidfd-backed process primitives in Ruby so processes can be opened, monitored, and waited on safely through the scheduler.

Why it matters:
Pidfds eliminate race conditions, improve cross-thread safety, and make process management reliably asynchronous. This enables safer job-runners, supervisors, and async orchestration patterns in Ruby.

[noteflakes]

See also ruby/ruby#15213.