cmake: add find_package(Boost) for WITH_CGAL mode

CGAL 6.x still unconditionally includes boost/config.hpp in its
config.h, so Boost headers are required even with CGAL_DISABLE_GMP/MPFR.

Use system Boost via find_package instead of the removed 211 MB tarball.
Tests-only mode remains completely dependency-free.

Install: brew install boost  (macOS)
         apt install libboost-dev  (Debian/Ubuntu)

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

code/CMakeLists.txt

@@ -85,12 +85,18 @@ endif()
 
 # ── Core CLI app (optional, requires CGAL + Viewer) ───────────────────────────
 if(WITH_CGAL)
+    # CGAL 6.x still needs Boost.Config headers unconditionally.
+    # Install via: brew install boost  (macOS)
+    #              apt install libboost-dev  (Debian/Ubuntu)
+    find_package(Boost REQUIRED)
+
     add_executable(${PROJECT_NAME} src/apps/v0/conformallab_cli.cpp)
     target_include_directories(${PROJECT_NAME} SYSTEM PRIVATE
         ${CMAKE_CURRENT_SOURCE_DIR}/deps/single_includes
         ${CMAKE_CURRENT_SOURCE_DIR}/deps/eigen-3.4.0/
         ${CMAKE_CURRENT_SOURCE_DIR}/deps/CGAL-6.1.1/include
-        ${CMAKE_CURRENT_SOURCE_DIR}/deps/libigl-2.6.0/include)
+        ${CMAKE_CURRENT_SOURCE_DIR}/deps/libigl-2.6.0/include
+        ${Boost_INCLUDE_DIRS})
     target_include_directories(${PROJECT_NAME} PRIVATE
         ${CMAKE_CURRENT_SOURCE_DIR}/include)
     target_compile_definitions(${PROJECT_NAME} PRIVATE