Function enforce_cuda_required 

pub fn enforce_cuda_required(context: &str, err: &XlogError)

Release-gate hardening: when XLOG_REQUIRE_CUDA=1, CUDA/test-context initialization failures must fail loudly instead of returning Err. Tests use let Ok(ctx) = ... else { return; } skip patterns that pass vacuously without a GPU; scripts/validate_release_gpu.sh exports this variable so a CPU-only machine can never satisfy the certification gate.