ci: add Codeberg CI (Woodpecker + Forgejo) and document CI/CD topology

Add a lightweight test-fast pipeline for the public Codeberg mirror so it
shows a build status, defined for both CI systems Codeberg offers:
- .woodpecker.yml            (Woodpecker, debian:bookworm)
- .forgejo/workflows/        (Forgejo Actions, node:20-bookworm, runner label docker)

Both run the pure-math suite only (Eigen vendored, GTest via FetchContent;
no CGAL/Boost), using public images and the docker runner label since the
private eulernest ci-cpp image and Pi runner are unreachable from Codeberg.

Docs:
- new doc/architecture/ci-cd.md: two-forge topology, runners, trigger
  keywords, mirror mechanism.
- fix stale "codeberg/main ... no CI" claim in contributing.md and bring
  the CI table up to date.
- link the new doc from the README doc index.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.forgejo/workflows/cpp-tests.yml

@@ -0,0 +1,65 @@
+# Forgejo Actions — Codeberg (https://codeberg.org)
+#
+# Codeberg counterpart of the eulernest Gitea pipeline
+# (.gitea/workflows/cpp-tests.yml).  Two differences are required because
+# Codeberg's shared runners are not the Pi runner:
+#
+#   * runs-on: docker            — Codeberg shared-runner label (not "eulernest")
+#   * image:   node:20-bookworm  — public image (the private git.eulernest.eu
+#                                  ci-cpp image is unreachable from Codeberg).
+#                                  node:20 ships the Node runtime that
+#                                  actions/checkout@v4 needs; build tools are
+#                                  apt-installed in the first step.
+#
+# Scope: "test-fast" only — the pure-math suite (Eigen vendored, GoogleTest via
+# FetchContent, no CGAL/Boost).  A red job here = a red Codeberg CI badge.
+
+name: C++ Tests (Codeberg)
+
+on:
+  push:
+    branches:
+      - main
+      - "claude/**"
+      - "feature/**"
+      - "review/**"
+  pull_request:
+
+jobs:
+  test-fast:
+    runs-on: docker
+    container:
+      image: node:20-bookworm
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install build deps (cmake, g++, make)
+        run: |
+          apt-get update -qq
+          apt-get install -y --no-install-recommends \
+            cmake g++ make git ca-certificates
+
+      - name: Configure (tests-only — Eigen + GoogleTest)
+        run: cmake -S code -B build -DCMAKE_BUILD_TYPE=Release
+
+      - name: Build
+        run: cmake --build build --target conformallab_tests -j$(nproc)
+
+      - name: Run tests
+        run: >
+          ctest --test-dir build
+          --output-on-failure
+          --output-junit test-results.xml
+
+      - name: Summary
+        if: always()
+        run: |
+          if [ -f build/test-results.xml ]; then f=build/test-results.xml; else f=test-results.xml; fi
+          if [ -f "$f" ]; then
+            total=$(grep -o 'tests="[0-9]*"'     "$f" | grep -o '[0-9]*' | head -1)
+            failed=$(grep -o 'failures="[0-9]*"' "$f" | grep -o '[0-9]*' | head -1)
+            skipped=$(grep -o 'skipped="[0-9]*"' "$f" | grep -o '[0-9]*' | head -1)
+            passed=$(( ${total:-0} - ${failed:-0} - ${skipped:-0} ))
+            echo "FAST  ▸ TOTAL ${total:-0} | PASSED $passed | FAILED ${failed:-0} | SKIPPED ${skipped:-0}"
+          fi