add clang format CI job

.github/workflows/formatting.yml

@@ -0,0 +1,43 @@
+name: code formatting
+
+on:
+  push:
+    branches:
+      - main
+      - devel
+  pull_request:
+
+
+jobs:
+  build:
+    name: Auto format with clang 18
+    runs-on: ubuntu-24.04
+
+    steps:
+    - name: checkout
+      uses: actions/checkout@v4
+    - name: install packages
+      run: sudo apt install clang-18
+    - name: run clang format
+      run: |
+        ./do_clang_format.sh
+    - name: check for differences
+      run: |
+        git diff >clang-format.patch
+        if [ $(wc -c <clang-format.patch) -ne 0 ] ; then
+           echo "there was a formatting issue."
+           cat /etc/os-release
+           echo "you may download the artifact and apply it with git apply"
+           cat clang-format.patch
+           exit 1
+        else
+           rm clang-format.patch
+        fi
+    - name: store patch as artifact if there is one
+      if: failure()
+      uses: actions/upload-artifact@v4
+      with:
+        name: clang-format.patch
+        path: clang-format.patch
+        if-no-files-found: ignore
+