UniversalHash solver backends (CPU/GPU).
The [Solver] trait provides a backend-agnostic interface for proof-of-work
computation. Enable feature flags to select backends:
cpu(default) — multi-threaded CPU solvercuda-backend— NVIDIA CUDA solvermetal-backend— Apple Metal solver (macOS only)gpu-opencl— OpenCL solver (AMD/Intel/Apple GPUs)gpu-wgpu— WGPU solver (cross-platform via WebGPU)