0.1.5rc2

Self-contained CUDA support on Linux, as an explicit opt-in choice. localm setup-llama --backend cuda now works on Linux the same way it already does on Windows: it fetches a compiled build plus the CUDA runtime libraries (cudart, cuBLAS), no CUDA Toolkit install needed. This is a release candidate to get real hardware confirmation before it becomes the default - Vulkan remains the recommended backend on Linux until that lands.