gcc -O0 keeps unused cst_test_util.h helpers that need os_*; clang GC'd
them so only the gcc CMake step failed. Prefer ghfast/ghproxy.net for the
llvm-mingw tarball (gitclone 500 / github unreachable on the runner).
clang under strict C11 left strcasestr undeclared (returns int → pointer
truncation → SIGSEGV). Parse Content-Length portably; keep -Werror on
make check. Drop Nix for windows-cross (root /nix fails) and use pinned
llvm-mingw UCRT instead; Alpine musl gets linux-headers for audit.h.
WHC