Function write_dev_auth_file 

pub fn write_dev_auth_file(
    data_dir: &Path,
    auth_key: &str,
    web_endpoint: &str,
    ws_endpoint: &str,
    ipc_endpoint: &str,
    ipc_token: &str,
    instance: &str,
    host_pid: u32,
) -> Result<PathBuf, String>

Write authkey.dev to data_dir. Returns the absolute file path on success. Errors are returned as strings — the caller in main.rs logs them at warn-level and continues; a missing dev file is not a fatal startup failure.