setup.sh/setup.ps1 now run install_llama_prebuilt.py --smoke-test on a freshly
source-built GPU binary and retry a CPU build if it loaded on CPU only. When a
GPU host's source build produces no binary, both scripts fall back to the CPU
prebuilt (--cpu-fallback) as a labelled last resort instead of leaving the host
without llama.cpp. setup.ps1 also guards an empty CUDA arch (no PTX-only binary,
#5854). The POSIX smoke-test exit code is captured set -e safe. Adds a fake
llama-server and an end-to-end spoof test that runs the real validate_server
against it with no GPU.
