docs + ci: update README, add ci-cpp Docker image, use container in workflow

README:
- Remove stale Boost mention (unused dep, removed yesterday)
- Document three build modes (tests-only / WITH_VIEWER / WITH_CGAL)
  with a comparison table and per-mode cmake commands
- Add project structure overview
- Fix C++20 → C++17 (actual standard used)
- Fix clone URL and getting-started commands
- Add CI section with Dockerfile build instructions

CI:
- Add .gitea/docker/Dockerfile.ci-cpp — ubuntu:22.04 with cmake,
  g++, git, and Node.js 20 pre-installed (Node.js needed for
  actions/checkout@v4 inside containers)
- Update cpp-tests.yml to use ci-cpp container instead of installing
  build tools on every run; add JUnit XML output and summary step

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/docker/Dockerfile.ci-cpp

@@ -0,0 +1,16 @@
+FROM ubuntu:22.04
+
+# Node.js 20 from NodeSource (Ubuntu Jammy ships v12 which is too old
+# for actions/checkout@v4 — static class blocks require Node.js >= 16).
+RUN apt-get update -qq && \
+    apt-get install -y --no-install-recommends \
+      curl ca-certificates && \
+    curl -fsSL https://deb.nodesource.com/setup_20.x | bash - && \
+    apt-get install -y --no-install-recommends \
+      nodejs \
+      cmake \
+      g++ \
+      git \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /workspace

.gitea/workflows/cpp-tests.yml

@@ -11,27 +11,32 @@ on:
 jobs:
   test:
     runs-on: eulernest
+    container:
+      image: git.eulernest.eu/conformallab/ci-cpp:latest
 
     steps:
       - uses: actions/checkout@v4
 
-      - name: Install build tools
-        run: |
-          sudo apt-get update -qq
-          sudo apt-get install -y --no-install-recommends \
-            cmake \
-            g++ \
-            git \
-            ca-certificates
-
-      - name: Configure (Release, tests enabled)
-        run: |
-          cmake -S code -B build \
-            -DCMAKE_BUILD_TYPE=Release \
-            -DBUILD_TESTING=ON
+      - name: Configure (tests-only mode)
+        run: cmake -S code -B build -DCMAKE_BUILD_TYPE=Release
 
       - name: Build test binary
         run: cmake --build build --target conformallab_tests -j$(nproc)
 
       - name: Run tests
-        run: ctest --test-dir build -R conformallab_tests --output-on-failure
+        run: >
+          ctest --test-dir build
+          --output-on-failure
+          --output-junit test-results.xml
+
+      - name: Show test summary
+        if: always()
+        run: |
+          if [ -f test-results.xml ]; then
+            total=$(grep -o 'tests="[0-9]*"'    test-results.xml | grep -o '[0-9]*' | head -1)
+            failed=$(grep -o 'failures="[0-9]*"' test-results.xml | grep -o '[0-9]*' | head -1)
+            skipped=$(grep -o 'skipped="[0-9]*"' test-results.xml | grep -o '[0-9]*' | head -1)
+            passed=$(( ${total:-0} - ${failed:-0} - ${skipped:-0} ))
+            echo ""
+            echo "TOTAL: ${total:-0} | PASSED: $passed | FAILED: ${failed:-0} | SKIPPED: ${skipped:-0}"
+          fi