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/newton_solver.hpp

@@ -465,7 +465,7 @@ inline NewtonResult newton_hyper_ideal(
 /// Solve the CP-Euclidean circle-packing problem: find ρ ∈ ℝ^F such that the
 /// per-face angle sums match φ_f at every free face.
 ///
-/// The CP-Euclidean energy (Bobenko-Pinkall-Springborn 2010 §6) is strictly
+/// The CP-Euclidean energy (Bobenko-Pinkall-Springborn 2015 §6) is strictly
 /// convex on its open domain of validity, so the Hessian H is PSD and the
 /// solution is unique up to the gauge mode pinned by `f_idx == −1`.
 /// `cp_euclidean_hessian` provides the analytic 2×2-per-edge formula
@@ -483,7 +483,7 @@ inline NewtonResult newton_hyper_ideal(
 /// \note  Unlike the Euclidean solver, the CP-Euclidean Hessian is exact
 ///        (analytic), so the SparseQR fallback only triggers in genuine
 ///        gauge-singular situations (no pinned face).
-/// \see   doc/architecture/phase-9a-validation.md §1 for the BPS-2010 mapping.
+/// \see   doc/architecture/phase-9a-validation.md §1 for the BPS-2015 mapping.
 inline NewtonResult newton_cp_euclidean(
     ConformalMesh&             mesh,
     std::vector<double>        x0,