v0.2.1 - Windows CUDA build

Windows now ships the CUDA build. It used to be Vulkan-only because nvcc cannot
cross-compile from Linux; it is built on Windows in CI now and folded into the
archive.

Windows (soat-miner_v0.2.1_Win64.zip) carries both backends. soat-miner.bat
picks by GPU: CUDA on 40-series and older (about 34% faster on Ada), Vulkan on
Blackwell and on AMD/Intel. The CUDA binary is a multi-arch fat build (Turing,
Ampere, Ada native, plus PTX for Hopper and Blackwell) with the CUDA runtime
linked statically, so there is no cudart64 DLL to chase and no Toolkit to
install. It was verified to run on a machine with no CUDA Toolkit present.

Measured CUDA hashrate at the 7.27 GB dataset: 4080 125.3, 4070 SUPER 97.9,
4060 Ti 63.5 MH/s.

No source change to the miner itself. No dev fee, still.