add GTest infrastructure and port first mathematical tests from Java

- Add Google Test via CMake FetchContent (v1.14.0)
- Add clausen.hpp: Clausen integral, Lobachevsky function, Im(Li2)
- Add hyper_ideal_utility.hpp: generalized and ideal-vertex hyperbolic
  tetrahedron volume formulas using Eigen for the 4x4 Gram determinant
- Port ClausenTest (5 tests) and HyperIdealUtilityTest (8 tests) from
  Java/JUnit — all 13 pass with same tolerances as the Java originals
- Fix pre-existing VIEWER/viewer case mismatch in CMakeLists.txt

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

code/.gitignore

@@ -1,5 +1,6 @@
 # Build artifacts
 build/
+build_release/
 *.o
 *.so
 *.dylib