Proper fork support for Fiber Scheduler (Fiber::Scheduler#process_fork).

[ioquatix]

Summary:
Define how fiber schedulers behave across fork: the child should start in a clean state, with hooks to reinitialize or discard scheduler data safely.

Why it matters:
fork + async currently work inconsistently. This project makes forking predictable, allowing libraries and apps to do post-fork setup (e.g., reconnect I/O, restart loops) correctly and safely.

[samuel-williams-shopify]

Proposed implementation: ruby/ruby#15342

cc @noteflakes