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The LLVM NSIS .exe /S silent install HANGS in the headless SYSTEM CI session (stuck >15min after download, blocking the single runner). Switch to the portable clang+llvm-21.1.2-x86_64-pc-windows-msvc.tar.xz (curl + Win11 tar -xf, strip 1) — deterministic, no installer. And make driver-build run the provision script itself (idempotent) so it self-provisions LLVM and never races a separate provision run. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>