A metadata match used to short-circuit straight to reuse, so a previously
installed CPU-only 'GPU' binary survived every rerun and restart -- the exact
'picks CPU forever' report. install_prebuilt now smoke-tests a matching GPU
install and reinstalls if it loads on CPU; setup.sh/setup.ps1 smoke-test a reused
source build on a GPU host and rebuild if it ran on CPU. Non-GPU installs keep
the fast path.