fix(citations): correct Kolpakov-Mednykh misattribution → Springborn 2008

arXiv:math/0603097 is Springborn 2008 ("A variational principle for weighted
Delaunay triangulations and hyperideal polyhedra"), not a Kolpakov-Mednykh paper.
The author pair Kolpakov & Mednykh has no joint publication from 2006; their
earliest collaboration is arXiv:1008.0312 (2010, on torus knots, unrelated).

The wrong author name was introduced during the Java→C++ port — the Java source
correctly links to math/0603097 without naming the authors; whoever ported it
invented "Kolpakov-Mednykh". The S1 citation audit (2026-05-31) then cemented
the error by adding the incorrect row to references.md.

Files corrected (7):
- code/include/hyper_ideal_utility.hpp
- code/include/hyper_ideal_functional.hpp
- code/tests/cgal/test_hyper_ideal_functional.cpp
- doc/math/references.md
- doc/roadmap/research-track.md
- doc/architecture/project-structure.md
- doc/api/tests.md

Also:
- doc/reviewer/math-derivation-citation-audit-2026-05-31.md: M1 post-correction noted
- doc/reviewer/finding-orchestration.md: lesson-learned section added (AI citation
  audits can introduce plausible-but-wrong attributions; human expert review required
  before CGAL submission)
- papers/MANUAL-DOWNLOAD.md: overview of papers requiring manual download (paywalled
  journals, TU Berlin theses, books)
- .gitignore: papers/*.pdf excluded (downloaded arXiv PDFs, not tracked)

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

doc/roadmap/research-track.md

@@ -145,10 +145,10 @@ The phase numbers match `doc/roadmap/phases.md`.
 ### Hyper-ideal volume formulas for 2- and 3-ideal-vertex faces (Phase 9b+, 🔲 planned)
 
 * **Mathematical sources:**
-  - **Kolpakov, A. & Mednykh, A.** (2012). *Spherical structures on torus
-    knots and links.* Sibirsk. Mat. Zh. 53(3), 535–541 — see the earlier
-    arXiv:math/0603097 for the one-ideal-vertex formula already implemented
-    as `calculateTetrahedronVolumeWithIdealVertexAtGamma`.
+  - **Springborn, B.** (2008). *A variational principle for weighted Delaunay
+    triangulations and hyperideal polyhedra.* J. Differential Geometry **78**(2),
+    333–367. arXiv:math/0603097 — the source of the one-ideal-vertex formula
+    already implemented as `calculateTetrahedronVolumeWithIdealVertexAtGamma`.
   - **Milnor, J.** (1982). *Hyperbolic geometry: The first 150 years.*
     Bull. Amer. Math. Soc. 6(1), 9–24.  → Volume of an ideal tetrahedron
     via Clausen function; this is the all-ideal case with 4 ideal vertices.
@@ -175,10 +175,10 @@ The phase numbers match `doc/roadmap/phases.md`.
 
 * **Acceptance criteria:**
   - Identify the correct formula for a hyper-ideal tetrahedron with exactly
-    2 ideal vertices from the literature (check Kolpakov-Mednykh generalisations
+    2 ideal vertices from the literature (check Springborn 2008 §3–4 generalisations
     and Vinberg orthoscheme decomposition).
   - Implement `calculateTetrahedronVolumeWithTwoIdealVertices(…)` analogous
-    to the existing Kolpakov-Mednykh function.
+    to the existing Springborn 2008 one-ideal-vertex function.
   - Implement `calculateTetrahedronVolumeWithThreeIdealVertices(…)` (one
     hyper-ideal + three ideal = fully cusp-like case).
   - Replace the `throw std::logic_error` in `face_energy()` with the correct