chore: translate all German text to English across code, docs, and CI

Unified the codebase language to English throughout. German text appeared
in code comments, test file headers, CI step names, and several markdown
documents. All natural-language text is now English; proper nouns
(Institut für Mathematik, Technische Universität Berlin) are unchanged.

Files changed:
- .gitea/workflows/cpp-tests.yml  — CI step names and job comments
- code/include/mesh_utils.hpp     — inline comment
- code/tests/cgal/CMakeLists.txt  — section comment block
- code/tests/cgal/test_geometry_utils.cpp — full file header + all test comments
- doc/math/references.md          — geometry-central section
- doc/math/validation.md          — Section 9 (geometry-central cross-validation)
- doc/roadmap/phases.md           — Optional geometry-central track (GC-1/2/3)

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

.gitea/workflows/cpp-tests.yml

@@ -11,8 +11,8 @@ on:
 
 # ─────────────────────────────────────────────────────────────────────────────
 # Job 1 — test-fast
-#   Pure-math tests (Clausen, ImLi₂, Hyper-ideal Geometrie).
-#   Kein CGAL, kein Boost. Nur Eigen + GTest. Läuft auf ALLEN Branches.
+#   Pure-math tests (Clausen, ImLi₂, Hyper-ideal geometry).
+#   No CGAL, no Boost. Eigen + GTest only. Runs on ALL branches.
 # ─────────────────────────────────────────────────────────────────────────────
 jobs:
   test-fast:
@@ -35,7 +35,7 @@ jobs:
           --output-on-failure
           --output-junit test-results.xml
 
-      - name: Zusammenfassung
+      - name: Summary
         if: always()
         run: |
           if [ -f test-results.xml ]; then
@@ -48,14 +48,14 @@ jobs:
 
 # ─────────────────────────────────────────────────────────────────────────────
 # Job 2 — test-cgal
-#   Vollständige CGAL-Test-Suite (Phase 3–7, 158 Tests).
-#   Läuft NUR bei Pull Requests (nicht bei direkten Pushes auf dev/main).
-#   Startet erst nach erfolgreichem test-fast.
+#   Full CGAL test suite (Phase 3–7, 158 tests).
+#   Runs ONLY on pull requests (not on direct pushes to dev/main).
+#   Starts only after test-fast succeeds.
 #
-#   Verwendet -DWITH_CGAL_TESTS=ON (nicht -DWITH_CGAL=ON), damit kein
-#   Viewer/GLFW gebaut wird — der CI-Container hat kein wayland-scanner.
+#   Uses -DWITH_CGAL_TESTS=ON (not -DWITH_CGAL=ON) to avoid building
+#   Viewer/GLFW — the CI container has no wayland-scanner.
 #
-#   Boost (libboost-dev) ist seit Image-Rebuild bereits im Container.
+#   Boost (libboost-dev) is already present in the container since the image rebuild.
 # ─────────────────────────────────────────────────────────────────────────────
   test-cgal:
     needs: test-fast
@@ -68,7 +68,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Configure (WITH_CGAL_TESTS — kein Viewer, kein wayland-scanner)
+      - name: Configure (WITH_CGAL_TESTS — no viewer, no wayland-scanner)
         run: cmake -S code -B build -DWITH_CGAL_TESTS=ON -DCMAKE_BUILD_TYPE=Release
 
       - name: Build CGAL-Tests
@@ -81,7 +81,7 @@ jobs:
           --output-on-failure
           --output-junit cgal-results.xml
 
-      - name: Zusammenfassung
+      - name: Summary
         if: always()
         run: |
           if [ -f cgal-results.xml ]; then