Fix CI: install llvm and set LIBCLANG path

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: cfis

.github/workflows/test.yml

@@ -17,8 +17,10 @@ jobs:
     steps:
       - uses: actions/checkout@v6
 
-      - name: Install libclang
-        run: sudo apt-get install -y libclang-dev
+      - name: Install LLVM and libclang
+        run: |
+          sudo apt-get install -y llvm libclang-dev
+          echo "LIBCLANG=$(llvm-config --libdir)/libclang.so" >> $GITHUB_ENV
 
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1