Fork children from a clean fiber

[samuel-williams-shopify]

Summary

• Replace the short-lived native thread used for Process.fork with a blocking Fiber.
• Keep fork on the caller's native thread for compatibility with native libraries such as gRPC.
• Preserve the clean child stack: CRuby promotes the current Fiber to the child root and clears its resuming Fiber after fork.
• Add a regression test proving objects retained only by the caller stack can be collected in the child.

CRuby behavior: https://bugs.ruby-lang.org/issues/15041

Testing

• ruby -Ilib .../sus test/async/container/forked.rb
• 30 tests, 60 assertions passed.
• Ruby syntax and git diff --check passed.