Thread safety task wait

[samuel-williams-shopify]

This PR refactors Task to use Promise for state management, achieving the goal that "a task should be a promise with attached computation and stop (cancellation) handling." The key changes include: moving @status logic into Promise using @resolved with consistent symbol states (:completed, :failed, :cancelled), renaming @finished to @promise, deriving running? state from @fiber&.alive?, and adding sophisticated exception handling to Promise#fulfill with proper non-local exit support. This maintains full backward compatibility while providing cleaner state management, comprehensive test coverage (47 new Promise tests), and improved code organization with zero breaking changes to the public API.

Types of Changes

• New feature.
• Performance improvement.

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.