Honor to_io-only redirection wrappers - #202

Would need a test, and a real-world use case

Added an end-to-end regression using an IO adapter that deliberately exposes only Ruby’s to_io conversion protocol around a tempfile. This is the concrete use case: lifecycle/instrumentation wrappers can keep their public surface narrow while still being valid redirection targets (and ChildProcess::Unix::IO#check_type already explicitly accepts them). Before this change assignment succeeds but start raises on the missing direct fileno; now the child output reaches the wrapped IO. The focused IO spec passes (11 examples).