fix: detect and install Clang 15-18 with LLVM fallback

- Tries apt clang-18/17/16/15 first (Ubuntu 22.04+)
- Falls back to LLVM repo install for Ubuntu 20.04
- Auto-detects installed clang version for CMake compiler flags
- Both CMake 3.28 (binary download) and C++23 compiler required

.gitea/workflows/ci.yml

@@ -15,7 +15,21 @@ jobs:
       - name: Install dependencies
         run: |
           apt-get update -qq
-          apt-get install -y -qq clang ninja-build curl
+          # Try clang-15+ first (Ubuntu 22.04+)
+          if apt-cache show clang-18 >/dev/null 2>&1; then
+            apt-get install -y -qq clang-18 ninja-build curl
+          elif apt-cache show clang-17 >/dev/null 2>&1; then
+            apt-get install -y -qq clang-17 ninja-build curl
+          elif apt-cache show clang-16 >/dev/null 2>&1; then
+            apt-get install -y -qq clang-16 ninja-build curl
+          elif apt-cache show clang-15 >/dev/null 2>&1; then
+            apt-get install -y -qq clang-15 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
+          fi
 
       - name: Install CMake 3.28
         run: |
@@ -24,9 +38,12 @@ jobs:
 
       - name: Build kappa
         run: |
+          CC=$(ls /usr/bin/clang-1[5-9] /usr/bin/clang-2[0-9] 2>/dev/null | head -1)
+          if [ -z "$CC" ]; then CC=/usr/bin/clang-18; fi
+          CXX="${CC}++"
           cmake -B build -G Ninja \
-            -DCMAKE_C_COMPILER=clang \
-            -DCMAKE_CXX_COMPILER=clang++
+            -DCMAKE_C_COMPILER="$CC" \
+            -DCMAKE_CXX_COMPILER="$CXX"
           cmake --build build
 
       - name: DSL and CLI tests