Replace platform-specific std::os::unix::io::FromRawFd with
Box<dyn std::io::Write + Send> return type. This compiles on
Windows, macOS, and Linux without unsafe code.
The closure now returns a boxed writer that is either:
- The cloned file handle (success case)
- A reopened file handle (clone failed)
- stderr (last resort, prevents panic)
huqiantao
27ca87251e