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>

doc/math/references.md

@@ -28,22 +28,22 @@ Java reference implementation: [github.com/varylab/conformallab](https://github.
 
 ## geometry-central cross-reference *(optional comparison track)*
 
-> Diese Referenzen beziehen sich auf eine alternative Implementierung desselben
-> mathematischen Problems. Sie sind keine Voraussetzung für conformallab++,
-> aber relevant für Kreuz-Validierung und mögliche algorithmische Adoptionen
-> (→ GC-1/2/3 im Phasen-Roadmap, → Abschnitt 9 in `validation.md`).
+> These references relate to an alternative implementation of the same
+> mathematical problem. They are not prerequisites for conformallab++,
+> but are relevant for cross-validation and possible algorithmic adoptions
+> (→ GC-1/2/3 in the phase roadmap, → Section 9 in `validation.md`).
 
-| Reference | Relevanz |
+| Reference | Relevance |
 |---|---|
-| **Gillespie, Springborn, Crane** — *Discrete Conformal Equivalence of Polyhedral Surfaces*, ACM SIGGRAPH 2021. DOI: [10.1145/3450626.3459763](https://doi.org/10.1145/3450626.3459763) | Implementiert in **geometry-central**. Erweitert Springborn 2020 um intrinsische Triangulierungen und Ptolemäische Flips. Löst dasselbe DCE-Problem wie conformallab++, aber mit anderem Algorithmus. |
-| **Sharp, Soliman, Crane** — *Navigating Intrinsic Triangulations*, ACM SIGGRAPH 2019 | Algorithmische Grundlage für `SignpostIntrinsicTriangulation` in geometry-central — relevant für GC-2 (optionales Pre-Conditioning). |
+| **Gillespie, Springborn, Crane** — *Discrete Conformal Equivalence of Polyhedral Surfaces*, ACM SIGGRAPH 2021. DOI: [10.1145/3450626.3459763](https://doi.org/10.1145/3450626.3459763) | Implemented in **geometry-central**. Extends Springborn 2020 with intrinsic triangulations and Ptolemaic flips. Solves the same DCE problem as conformallab++, but with a different algorithm. |
+| **Sharp, Soliman, Crane** — *Navigating Intrinsic Triangulations*, ACM SIGGRAPH 2019 | Algorithmic basis for `SignpostIntrinsicTriangulation` in geometry-central — relevant for GC-2 (optional pre-conditioning). |
 
-**Hinweis zu Springborn 2020:**  
-Das Papier *"Ideal Hyperbolic Polyhedra and Discrete Uniformization"*
-(Springborn, Discrete & Computational Geometry 2020) ist **in conformallab++
-bereits implementiert** — es ist die direkte Referenz für den HyperIdeal-Geometriemodus
-(`hyper_ideal_geometry.hpp`). Die geometry-central Implementierung (Gillespie 2021)
-baut auf diesem Papier auf und ergänzt es um Ptolemäische Flips.
+**Note on Springborn 2020:**  
+The paper *"Ideal Hyperbolic Polyhedra and Discrete Uniformization"*
+(Springborn, Discrete & Computational Geometry 2020) is **already implemented in
+conformallab++** — it is the direct reference for the HyperIdeal geometry mode
+(`hyper_ideal_geometry.hpp`). The geometry-central implementation (Gillespie 2021)
+builds on this paper and augments it with Ptolemaic flips.
 
 ---