fix(citations): correct Meyerhoff/Ushijima misattribution

DOI 10.1007/0-387-29555-0_13 is Ushijima (sole author) —
"A Volume Formula for Generalised Hyperbolic Tetrahedra",
in: Prékopa & Molnár (eds.), Non-Euclidean Geometries,
Springer 2006. arXiv: math/0309216.

The references.md entry was wrong on all three counts:
  - Author:  "Meyerhoff, Ushijima" → Ushijima only
  - Title:   "A Note on the Dirichlet Domain" → entirely different title
  - Book:    "The Epstein Birthday Schrift" → Non-Euclidean Geometries

Same error pattern as the Kolpakov-Mednykh fix: the Java source
links only to a DOI without naming authors; a wrong name was
invented during the C++ port.

Files corrected: hyper_ideal_utility.hpp, hyper_ideal_functional.hpp,
references.md, tests.md, project-structure.md, finding-orchestration.md,
math-derivation-citation-audit.md, MANUAL-DOWNLOAD.md

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

doc/api/tests.md

@@ -7,7 +7,7 @@ Pure-math tests, only Eigen required. Covers Java utilities ported in Phase 1–
 | File | What it tests |
 |---|---|
 | `test_clausen.cpp` | Clausen Cl₂, Lobachevsky Л, ImLi₂ — values at known points |
-| `test_hyper_ideal_utility.cpp` | Tetrahedron volumes (Meyerhoff / Springborn 2008) + Java golden-value oracle (Clausen/Л/ImLi₂, ζ₁₃/₁₄/₁₅/ζ, both volume formulas) |
+| `test_hyper_ideal_utility.cpp` | Tetrahedron volumes (Ushijima 2006 / Springborn 2008) + Java golden-value oracle (Clausen/Л/ImLi₂, ζ₁₃/₁₄/₁₅/ζ, both volume formulas) |
 | `test_matrix_utility.cpp` | Matrix helpers |
 | `test_surface_curve_utility.cpp` | Surface curve utilities |
 | `test_discrete_elliptic_utility.cpp` | Discrete elliptic functions |