Add build.rs that compiles .cu kernels to cubins at cargo build time via nvcc.
Replace OnceLock<Ptx> + compile_ptx_for_device() in gpu_action_selector.rs with
include_bytes! + Ptx::from_binary() loading from the precompiled cubin.
- build.rs gracefully skips when nvcc is not available (non-CUDA builds)
- CUDA_COMPUTE_CAP env var controls target GPU arch (86=RTX 3050, 90=H100)
- common_device_functions.cuh prepended to kernel sources at build time
- Zero runtime compiler invocations for epsilon_greedy kernel
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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