Summary

Implements Fiber-Promise merge proposal from #9877

Problem

When a Fiber forks work then awaits a Promise, we have unnecessary allocations + indirection.

Solution

Add Promise.become method to link fibers to promises without extra allocations.

Changes

• Add Promise.become for fiber linking
• Add Fiber.toPromise for direct conversion
• Include usage examples and documentation

Benefits

• Reduced allocations in async workflows
• Simpler mental model
• Better performance for promise-heavy code

/claim #9877