Preserve underlying exception traceback on Err.unwrap

[katunilya]

Closes #3

Summary:

• chain ErrUnwrapError from stored BaseException values
• retain existing behavior for non-exception Err values
• cover synchronous and FutureResult unwrap paths, including BaseException subclasses

Verification:

• 44 tests passed with 100% coverage
• Ruff check and format passed
• Ty check passed with three pre-existing unused-ignore warnings
• MyPy passed