`Async.unsupervised` — explicit `fiber.join()` semantics

[rcardin]

Parent

#302

What to build

Add tests to AsyncUnsupervisedSpec.scala (or extend AsyncSpec.scala) that verify fiber.join() and fiber.value behavior inside Async.unsupervised.

fiber.join() is the only way to observe a fiber's exception in an unsupervised scope. Its contract must hold: rethrows on failure, returns normally on success, returns silently on cancellation.

No implementation changes — tests only, building on top of the core scope introduced in #303.

Acceptance criteria

• Test: fork a fiber that throws; call fiber.join() inside the block; assert the exception propagates to the caller of Async.unsupervised
• Test: fork a fiber that returns a value; call fiber.join() then fiber.value (or use fiber.value directly); assert the correct value is returned
• Test: fork a fiber, cancel it, call fiber.join(); assert no exception is raised (consistent with existing supervised Fiber.join() contract)
• Tests follow the Async.run { Async.unsupervised { ... } } pattern established in AsyncSpec.scala

Blocked by

• Implement Async.unsupervised scope — core lifecycle #303