docs(references): M1/M2/M4 citation fixes (audit quick-wins)

M1: Add two missing references used in hyper_ideal_utility:
  - Kolpakov, Mednykh (2006, arXiv math/0603097) — tetrahedron volume w/ one ideal vertex
  - Meyerhoff, Ushijima (2006) — tetrahedron volume w/ three ideal vertices

M2: Clarify BPS publication year: Geometry & Topology 2015 (arXiv 2010)
  - Update references.md to note "first posted 2010"
  - Normalize all code comments from "BPS-2010" → "BPS-2015" (published version)

M4: Standardize citation format in code comments
  - Normalize all "Luo (2004)" / "Luo-2004" / "Luo's 2004" → "Luo 2004"
  - Matches references.md convention: Author Year (no parens/dashes)

282/282 tests pass. Addresses M1, M2, M4 from math-derivation-citation audit.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

code/include/CGAL/Discrete_inversive_distance.h

@@ -8,7 +8,7 @@
 \ingroup PkgConformalMapRef
 
 User-facing entry for the **vertex-based** inversive-distance circle-
-packing functional of Luo (2004), with the Bowers-Stephenson (2004)
+packing functional of Luo 2004, with the Bowers-Stephenson (2004)
 initialisation.  See `inversive_distance_functional.hpp` for the
 underlying algorithm and `doc/roadmap/research-track.md` (item 9a.2)
 for the research-track classification — this functional has **no Java
@@ -98,7 +98,7 @@ struct Default_inversive_distance_traits<CGAL::Surface_mesh<typename K::Point_3>
 /*!
 \ingroup PkgConformalMapRef
 
-Compute the Luo-2004 vertex-based inversive-distance circle packing of `mesh`.
+Compute the Luo 2004 vertex-based inversive-distance circle packing of `mesh`.
 
 The per-edge constant `I_ij` is computed once at the start from the input
 3-D geometry via the Bowers-Stephenson identity