Function backup_write_files 

pub fn backup_write_files(
    cwd: &Path,
    files: &[(&Path, &str, &WritePolicy)],
) -> Result<()>

Back up files, write new content atomically, and finalize the backup session. All originals are saved before any writes begin, ensuring consistency.

Each element is (path, content, policy) where path is the file to write, content is the new file content, and policy controls write transformations.