Add sandbox_c cmake project + ccache workflow for pool testing

.gitea/workflows/sandbox-cmake.yml

@@ -0,0 +1,61 @@
+name: Sandbox - cmake + ccache on the pool
+on:
+  push:
+    branches: [ci/sandbox]
+  workflow_dispatch:
+
+env:
+  # Inside the workspace so actions/cache can store it. The path is identical on
+  # every run, which matters because ccache hashes the compiler command line.
+  CCACHE_DIR: ${{ github.workspace }}/.ccache
+
+jobs:
+  build:
+    runs-on: cmake
+    defaults:
+      run:
+        shell: bash
+        working-directory: sandbox_c
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Toolchain
+        run: |
+          set -euo pipefail
+          command -v cmake >/dev/null && command -v ccache >/dev/null && exit 0
+          apt-get update -qq
+          apt-get install -y -qq --no-install-recommends \
+            cmake ninja-build gcc libc6-dev ccache ca-certificates >/dev/null
+          cmake --version | head -1
+          ccache --version | head -1
+
+      # Cache the compiler cache, never build/: CMake bakes absolute paths into
+      # CMakeCache.txt, so a restored build tree breaks if the workspace moves.
+      - name: Cache ccache
+        uses: actions/cache@v4
+        with:
+          path: ${{ env.CCACHE_DIR }}
+          key: ccache-${{ hashFiles('sandbox_c/**') }}
+          restore-keys: ccache-
+
+      - name: ccache before build
+        run: ccache --show-stats | grep -E "cache (hits|misses|read)" || ccache --show-stats
+
+      - name: Configure
+        run: |
+          cmake -S . -B build -G Ninja \
+            -DCMAKE_C_COMPILER_LAUNCHER=ccache \
+            -DCMAKE_BUILD_TYPE=Release
+
+      - name: Build
+        run: cmake --build build --parallel
+
+      - name: Test
+        run: ctest --test-dir build --output-on-failure
+
+      - name: ccache after build
+        if: always()
+        run: |
+          ccache --show-stats
+          set -o pipefail
+          ctest --test-dir build 2>&1 | grep -E "tests passed"