fix: install libc++ alongside clang for <format> support

Clang 16 on Ubuntu 22.04 uses libstdc++ 12 which lacks <format>.
Install libc++-dev and pass -stdlib=libc++ to CMake.

.gitea/workflows/ci.yml

@@ -15,20 +15,20 @@ jobs:
       - name: Install dependencies
         run: |
           apt-get update -qq
-          # Try clang-15+ first (Ubuntu 22.04+)
+          # Clang needs libc++ for <format> on Ubuntu 22.04 (libstdc++ is pre-GCC-13)
           if apt-cache show clang-18 >/dev/null 2>&1; then
-            apt-get install -y -qq clang-18 ninja-build curl
+            apt-get install -y -qq clang-18 libc++-18-dev libc++abi-18-dev ninja-build curl
           elif apt-cache show clang-17 >/dev/null 2>&1; then
-            apt-get install -y -qq clang-17 ninja-build curl
+            apt-get install -y -qq clang-17 libc++-17-dev libc++abi-17-dev ninja-build curl
           elif apt-cache show clang-16 >/dev/null 2>&1; then
-            apt-get install -y -qq clang-16 ninja-build curl
+            apt-get install -y -qq clang-16 libc++-16-dev libc++abi-16-dev ninja-build curl
           elif apt-cache show clang-15 >/dev/null 2>&1; then
-            apt-get install -y -qq clang-15 ninja-build curl
+            apt-get install -y -qq clang-15 libc++-15-dev libc++abi-15-dev ninja-build curl
           else
             # Ubuntu 20.04: install from LLVM repo
             apt-get install -y -qq wget lsb-release
             wget -qO- https://apt.llvm.org/llvm.sh | bash -s -- 18
-            apt-get install -y -qq ninja-build curl
+            apt-get install -y -qq libc++-18-dev libc++abi-18-dev ninja-build curl
           fi
 
       - name: Install CMake 3.28
@@ -44,7 +44,8 @@ jobs:
           echo "CC=$CC CXX=$CXX"
           cmake -B build -G Ninja \
             -DCMAKE_C_COMPILER="$CC" \
-            -DCMAKE_CXX_COMPILER="$CXX"
+            -DCMAKE_CXX_COMPILER="$CXX" \
+            -DCMAKE_CXX_FLAGS="-stdlib=libc++"
           cmake --build build
 
       - name: DSL and CLI tests