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/CMakeLists.txt

@@ -22,14 +22,31 @@ endif()
 if(CMAKE_CXX_COMPILER_ID MATCHES "Clang|GNU")
   add_compile_options(-Wall -Wextra -Wpedantic)
   
-  # AddressSanitizer nur im Debug
-  if(CMAKE_BUILD_TYPE STREQUAL "Debug")
+  # AddressSanitizer nur im Debug (nicht für Tests, da gtest_discover_tests
+  # die Binary zur Buildzeit ausführt und ASan dabei hängt)
+  if(CMAKE_BUILD_TYPE STREQUAL "Debug" AND NOT BUILD_TESTING)
     add_compile_options(-fsanitize=address -fno-omit-frame-pointer)
     add_link_options(-fsanitize=address)
-  endif() 
+  endif()
 endif()
 
 
+# ----- Testing (Google Test via FetchContent) -----
+include(FetchContent)
+include(CTest)
+enable_testing()
+
+FetchContent_Declare(
+    googletest
+    GIT_REPOSITORY https://github.com/google/googletest.git
+    GIT_TAG        v1.14.0
+)
+set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+set(INSTALL_GTEST          OFF CACHE BOOL "" FORCE)
+set(BUILD_GMOCK            OFF CACHE BOOL "" FORCE)
+FetchContent_MakeAvailable(googletest)
+# --------------------------------------------------
+
 add_subdirectory(deps)
 add_subdirectory(deps/glfw-3.4)
 
@@ -38,7 +55,7 @@ add_library(glad STATIC ${CMAKE_CURRENT_SOURCE_DIR}/deps/libigl-glad/src/glad.c)
 target_include_directories(glad PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/deps/libigl-glad/include)
 # --------- VIEWER Lib (separat → nur bei src/viewer/* Änderung gebaut!) ---------
 add_library(viewer STATIC src/viewer/simple_viewer.cpp)
-target_include_directories(VIEWER PUBLIC
+target_include_directories(viewer PUBLIC
   ${CMAKE_CURRENT_SOURCE_DIR}/deps/libigl-2.6.0/include
   ${CMAKE_CURRENT_SOURCE_DIR}/include  
   ${CMAKE_CURRENT_SOURCE_DIR}/deps/libigl-glad/include
@@ -52,7 +69,7 @@ target_include_directories(${PROJECT_NAME} SYSTEM PRIVATE
   ${CMAKE_CURRENT_SOURCE_DIR}/deps/eigen-3.4.0/
   ${CMAKE_CURRENT_SOURCE_DIR}/deps/CGAL-6.1.1/include
   ${CMAKE_CURRENT_SOURCE_DIR}/deps/boost_1_90_0/
-  ${CMAKE_CURRENT_SOURCE_DIR}/deps/libigl-2.6.0/include  # Core libigl
+  ${CMAKE_CURRENT_SOURCE_DIR}/deps/libigl-2.6.0/include  
 )
 target_include_directories(${PROJECT_NAME} PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/include)
 #-------- Compiler-Definitionen ---------
@@ -61,5 +78,5 @@ target_link_libraries(${PROJECT_NAME} PRIVATE viewer)
 #--------- Output-Ordner ---------
 set_target_properties(${PROJECT_NAME} PROPERTIES RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/bin)
 
-
+add_subdirectory(tests)