Borrowing Io.Dir.handle into an Io.File for the post-rename directory fsync assumes the handle is an fsync-able fd. Where the backend opens directories O_PATH (observed as EBADF panics under the nixery CI runner; debug builds panic in the std errno path), that assumption fails. syncDir reopens "." beneath the directory as a real read-only fd, syncs through std.Io, and counts the same .sync fault ordinal as the path it replaces. All five borrow sites converted.