Kernels with USM pointer arguments fail on the OpenCL CPU device #62

Launching a kernel that takes a UsmBox fails on the Intel OpenCL CPU device:

Error: Exception { what: "opencl backend failed with error: 11 (UR_RESULT_ERROR_INVALID_MEM_OBJECT)" }

Reproduce on a machine whose only SYCL device is a CPU (oneAPI 2026.1.1, main at db1cd36):

source /opt/intel/oneapi/setvars.sh
cargo run -p sycl-rs --example kernel_launch

Cause: USM pointers go through the byte form of raw_kernel_arg, which the spec reserves for non-pointer values. Level Zero accepts this, but OpenCL reads the bytes as a buffer handle.