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