## Summary
- Fix double-close of file descriptor when using `&>` redirect with
shell builtin commands
- Add `dupeRef()` helper for cleaner reference counting semantics
- Add tests for `&>` and `&>>` redirects with builtins
## Test plan
- [x] Added tests in `test/js/bun/shell/file-io.test.ts` that reproduce
the bug
- [x] All file-io tests pass
## The Bug
When using `&>` to redirect both stdout and stderr to the same file with
a shell builtin command (e.g., `pwd &> file.txt`), the code was creating
two separate `IOWriter` instances that shared the same file descriptor.
When both `IOWriter`s were destroyed, they both tried to close the same
fd, causing an `EBADF` (bad file descriptor) error.
```javascript
import { $ } from "bun";
await $`pwd &> output.txt`; // Would crash with EBADF
```
## The Fix
1. Share a single `IOWriter` between stdout and stderr when both are
redirected to the same file, with proper reference counting
2. Rename `refSelf` to `dupeRef` for clarity across `IOReader`,
`IOWriter`, `CowFd`, and add it to `Blob` for consistency
3. Fix the `Body.Value` blob case to also properly reference count when
the same blob is assigned to multiple outputs
🤖 Generated with [Claude Code](https://claude.com/claude-code)
---------
Co-authored-by: Claude Latest model <noreply@anthropic.com>
