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/tests/CMakeLists.txt

@@ -0,0 +1,17 @@
+add_executable(conformallab_tests
+    test_clausen.cpp
+    test_hyper_ideal_utility.cpp
+)
+
+target_include_directories(conformallab_tests SYSTEM PRIVATE
+    ${CMAKE_SOURCE_DIR}/deps/eigen-3.4.0
+)
+
+target_include_directories(conformallab_tests PRIVATE
+    ${CMAKE_SOURCE_DIR}/include
+)
+
+target_link_libraries(conformallab_tests PRIVATE GTest::gtest_main)
+
+include(GoogleTest)
+gtest_discover_tests(conformallab_tests DISCOVERY_TIMEOUT 60)