Function check_deferred_potrf_info 

pub fn check_deferred_potrf_info(
    stream: &Arc<CudaStream>,
    info_dev: &CudaSlice<i32>,
) -> Result<(), String>

Download the POTRF deferred info scalar and return an error if non-zero.

Called once at end-of-fit (or whenever the convergence loop exits) to surface any factorization failure that was deferred device-side by potrf_in_place_reuse.