The post-download llama-quantize / llama-server smoke test JIT-compiles CUDA kernels on the first GPU forward pass and stalls every install and update by minutes on Blackwell (sm_100). Gate it behind _RUN_STAGED_PREBUILT_VALIDATION, disabled for now, keeping the smoke test and the source-build fallback it triggers fully intact so it can be restored by flipping the flag to True.
Hashless external prebuilts (e.g. lemonade) are not in the approved-sha256 manifest and rely on the functional smoke test as their only integrity gate, so they are always validated regardless of the flag; only approved bundles, already proven by the sha256 manifest, skip it.
The sha256 archive verification and the static Linux/macOS preflights are unchanged and still run for every install.
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