ci(macos): pin xlings llvm@20.1.7 (llvm 22.1.8 libc++ ABI link breakage)

xlings' default llvm moved to 22.1.8, whose libc++ headers reference
std::__1::__hash_memory — undefined at link on macOS (ld64.lld), failing
any gtest-using e2e (78_test_main_combinations). Pin the known-good
20.1.7 first, keep latest as fallback. Real fix (link against the
toolchain's own libc++) belongs to the hermetic-toolchain-link track
(.agents/docs/2026-07-07-hermetic-toolchain-link-model-design.md).
The version-discovery from the previous commit stays (picks whatever is
installed).

Author: Sunrisepeak

.github/workflows/ci-macos.yml

@@ -58,7 +58,10 @@ jobs:
 
       - name: Install LLVM via xlings
         run: |
-          xlings install llvm -y || xlings install llvm@20.1.7 -y
+          # Pin 20.1.7: llvm 22.1.8's libc++ headers reference __hash_memory,
+          # absent from the libc++ the link resolves on macOS (ld64.lld
+          # undefined symbol; hermetic-link follow-up tracks the real fix).
+          xlings install llvm@20.1.7 -y || xlings install llvm -y
           # Verify clang++ is available
           LLVM_ROOT=$(find "$HOME/.xlings" -path "*/xpkgs/xim-x-llvm/*/bin/clang++" | head -1 | xargs dirname | xargs dirname)
           echo "LLVM_ROOT=$LLVM_ROOT"