Install LLVM explicitly in sanitizer and assertions CI jobs

Use the official LLVM apt script to install clang rather than relying
on whichever version happens to ship on the runner. The version is
configured via a workflow-level LLVM_VERSION env variable (currently 21)
so bumping is a one-line change.

The assertions job also installs the matching versioned libc++-dev and
libc++abi-dev packages.

Remove CMAKE_CXX_COMPILER from the ci-assertions preset so the CXX
environment variable is respected by CMake in both CI and local use.

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

.github/workflows/main.yml

@@ -13,6 +13,9 @@ on:
     branches:
       - "*"
 
+env:
+  LLVM_VERSION: 21
+
 jobs:
   build:
     runs-on: ${{ matrix.os }}
@@ -261,7 +264,6 @@ jobs:
       contents: read
 
     env:
-      CXX: clang++
       ASAN_OPTIONS: strict_string_checks=1:detect_stack_use_after_return=1:check_initialization_order=1:strict_init_order=1:detect_leaks=1
       UBSAN_OPTIONS: print_stacktrace=1
 
@@ -278,6 +280,13 @@ jobs:
         git config --global user.email "41898282+github-actions[bot]@users.noreply.github.com"
         git config --global user.name "github-actions[bot]"
 
+    - name: Install LLVM
+      run: |
+        wget -qO /tmp/llvm.sh https://apt.llvm.org/llvm.sh
+        chmod +x /tmp/llvm.sh
+        sudo /tmp/llvm.sh $LLVM_VERSION
+        echo "CXX=clang++-$LLVM_VERSION" >> "$GITHUB_ENV"
+
     - name: Install system dependencies
       run: |
         sudo apt-get update
@@ -351,8 +360,10 @@ jobs:
     permissions:
       contents: read
 
-    env:
-      CXX: clang++
+    strategy:
+      fail-fast: false
+      matrix:
+        release: [stable, HEAD]
 
     steps:
     - uses: actions/checkout@v6
@@ -362,16 +373,23 @@ jobs:
         git config --global user.email "41898282+github-actions[bot]@users.noreply.github.com"
         git config --global user.name "github-actions[bot]"
 
+    - name: Install LLVM
+      run: |
+        wget -qO /tmp/llvm.sh https://apt.llvm.org/llvm.sh
+        chmod +x /tmp/llvm.sh
+        sudo /tmp/llvm.sh $LLVM_VERSION
+        echo "CXX=clang++-$LLVM_VERSION" >> "$GITHUB_ENV"
+
     - name: Install system dependencies
       run: |
         sudo apt-get update
-        sudo apt-get install -y ccache libc++-dev libc++abi-dev
+        sudo apt-get install -y ccache libc++-$LLVM_VERSION-dev libc++abi-$LLVM_VERSION-dev
 
     - name: Generate cache key
       shell: cmake -P {0}
       run: |
         string(TIMESTAMP current_date "%Y-%m-%d-%H;%M;%S" UTC)
-        file(APPEND "$ENV{GITHUB_ENV}" "CACHE_KEY=assertions_HEAD\n")
+        file(APPEND "$ENV{GITHUB_ENV}" "CACHE_KEY=assertions_${{ matrix.release }}\n")
         file(APPEND "$ENV{GITHUB_ENV}" "CACHE_TIMESTAMP=${current_date}\n")
 
     - name: Update the cache (ccache)
@@ -398,7 +416,7 @@ jobs:
 
     - name: Configure the project
       run: cmake --preset=ci-assertions
-        -Dsleigh_RELEASE_TYPE=HEAD
+        -Dsleigh_RELEASE_TYPE=${{ matrix.release }}
         -Dsleigh_BUILD_DOCUMENTATION=OFF
 
     - name: Build the project

CMakePresets.json

@@ -119,7 +119,6 @@
       "binaryDir": "${sourceDir}/build/assertions",
       "inherits": ["ci-unix", "dev-mode"],
       "cacheVariables": {
-        "CMAKE_CXX_COMPILER": "clang++",
         "CMAKE_BUILD_TYPE": "Assertions",
         "CMAKE_CXX_FLAGS_ASSERTIONS": "-O0 -g -stdlib=libc++ -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG -fstack-protector-strong",
         "CMAKE_EXE_LINKER_FLAGS_ASSERTIONS": "-stdlib=libc++ -lc++abi",