Add CTest coverage reporting

Instrument libakgl in opt-in coverage builds, use CTest fixtures to reset counters and emit Cobertura XML and detailed HTML reports, and upload those reports from Gitea CI. Document the local coverage workflow for contributors.

Co-authored-by: Codex (GPT-5) <noreply@openai.com>
Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>

.gitea/workflows/ci.yaml

@@ -14,7 +14,7 @@ jobs:
         run: |
           sudo apt-get update -y
           sudo apt-get install -y \
-            cmake doxygen gcc pkg-config \
+            cmake doxygen gcc gcovr pkg-config \
             libasound2-dev libfreetype-dev libharfbuzz-dev \
             libpng-dev libtiff-dev libwebp-dev \
             libudev-dev libx11-dev libxcursor-dev libxext-dev \
@@ -22,7 +22,9 @@ jobs:
             libxss-dev libxtst-dev
       - name: Configure and build
         run: |
-          cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug
+          cmake -S . -B build \
+            -DCMAKE_BUILD_TYPE=Debug \
+            -DAKGL_COVERAGE=ON
           cmake --build build --parallel
       - name: Build API documentation
         run: doxygen Doxyfile
@@ -43,6 +45,13 @@ jobs:
           detailed_summary: true
           include_passed: true
           fail_on_failure: 'true'
+      - name: Upload coverage reports
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: code-coverage
+          path: build/coverage/
+          if-no-files-found: warn
 
   mutation_test:
     runs-on: ubuntu-latest