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>

code/tests/cgal/CMakeLists.txt

@@ -44,10 +44,10 @@ add_executable(conformallab_cgal_tests
     #             period matrix, fundamental domain, tiling
     test_phase7.cpp
 
-    # ── Java-Parität: Geometrie-Utility-Tests ─────────────────────────────────
-    #    Portiert aus CuttinUtilityTest, UnwrapUtilityTest,
-    #    ConvergenceUtilityTests, HomologyTest (Tests 1–6).
-    #    Test 7 (Genus-2-Homologie) als GTEST_SKIP-Stub bis Phase 8.
+    # ── Java parity: geometry utility tests ──────────────────────────────────
+    #    Ported from CuttinUtilityTest, UnwrapUtilityTest,
+    #    ConvergenceUtilityTests, HomologyTest (tests 1–6).
+    #    Test 7 (genus-2 homology) as GTEST_SKIP stub until Phase 8.
     test_geometry_utils.cpp
 
     # ── Scalability smoke tests ────────────────────────────────────────────────