ci: test building against a system uchardet

.github/workflows/test-system-uchardet.yml

@@ -0,0 +1,70 @@
+name: Test (system uchardet)
+
+# Exercises the `system-uchardet` feature option (see meson.options): build
+# cChardet against a system uchardet built from git and confirm the resulting
+# extension links the shared library instead of the bundled sources. The
+# regular Tests workflow only covers the bundled default.
+on:
+  pull_request:
+  push:
+    branches:
+      - master
+
+jobs:
+  system-uchardet:
+    name: Build against system uchardet (from git)
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          # meson's files() validates the vendored uchardet sources exist at
+          # configure time even for a system build, so the submodule still has
+          # to be checked out.
+          submodules: recursive
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+
+      - name: Build and install uchardet from git
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y --no-install-recommends cmake ninja-build pkg-config
+          git clone --depth 1 \
+            https://gitlab.freedesktop.org/uchardet/uchardet.git /tmp/uchardet-src
+          cmake -S /tmp/uchardet-src -B /tmp/uchardet-build \
+            -G Ninja \
+            -DCMAKE_BUILD_TYPE=Release \
+            -DCMAKE_INSTALL_PREFIX=/usr \
+            -DBUILD_BINARY=OFF
+          sudo cmake --build /tmp/uchardet-build --target install
+          sudo ldconfig
+
+      - name: Confirm the system uchardet exposes the candidate API
+        run: |
+          pkg-config --exists uchardet
+          echo "system uchardet version: $(pkg-config --modversion uchardet)"
+          # `enabled`/`auto` reject a uchardet without this symbol; the last
+          # tagged release still reports 0.0.8 but only the git build ships it.
+          grep -q uchardet_get_n_candidates /usr/include/uchardet/uchardet.h
+
+      - name: Build cChardet against the system uchardet
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install pytest
+          python -m pip install . \
+            --config-settings=setup-args=-Dsystem-uchardet=enabled
+
+      - name: Verify the extension links the system libuchardet
+        run: |
+          so=$(python -c "import cchardet._cchardet as m; print(m.__file__)")
+          echo "built extension: $so"
+          ldd "$so"
+          # Fail if it did not link the shared system library -- i.e. it
+          # silently fell back to compiling the bundled sources.
+          ldd "$so" | grep -q libuchardet
+
+      - name: Run tests
+        run: python -m pytest src/tests