Add GitHub Actions workflow for coverage reporting

.github/workflows/coverage.yml

@@ -0,0 +1,38 @@
+name: Coverage
+
+on: [push]
+
+jobs:
+  coverage:
+    name: Coverage
+    runs-on: ubuntu-24.04
+    env:
+      MRUBY_CONFIG: ci/gcc-clang
+      CC: gcc
+      CXX: g++
+      LD: gcc
+      CFLAGS: -O0 -g --coverage
+      LDFLAGS: --coverage
+    steps:
+      - name: "Checkout ${{ github.ref }} ( ${{ github.sha }} )"
+        uses: actions/checkout@v4
+      - name: Ruby version
+        run: ruby -v
+      - name: Compiler version
+        run: ${{ env.CC }} --version
+      - name: Build and test
+        run: rake test
+      - name: Install gcovr
+        run: pip install gcovr
+      - name: Generate coverage report
+        run: |
+          echo "# Coverage report" > $GITHUB_STEP_SUMMARY
+          echo \`\`\` >> $GITHUB_STEP_SUMMARY
+          gcovr -e ".*_test.c" -e ".*/test/.*" -s --html-details -o coverage/ build >> $GITHUB_STEP_SUMMARY
+          echo \`\`\` >> $GITHUB_STEP_SUMMARY
+      - name: Upload coverage report
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage-${{ github.sha }}
+          path: coverage/
+          retention-days: 14