Replace hand-rolled shared-memory reduction kernels (reduceBlock/reduceBlocks/
reduceKernel) and the global device variable retirementCount with a unified
CUB/hipCUB DeviceReduce::Reduce path for CUDA/HIP and sycl::reduction for SYCL.
No small/large split is needed: both CUB and sycl::reduction handle arbitrary
object sizes internally.
Old implementations preserved as sum_gpu_old / sumD_gpu_old etc. in the
original files for regression testing on GPU hardware.
Also add CLAUDE.md with build, test, and architecture guidance.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Peter Boyle