fix(deps): extract CGAL for WITH_CGAL_TESTS=ON

deps/CMakeLists.txt only extracted CGAL when WITH_CGAL=ON.
With -DWITH_CGAL_TESTS=ON the CGAL include path was never populated,
causing fatal error: CGAL/Simple_cartesian.h: No such file or directory.

Fix: extract CGAL-6.1.1 for both WITH_CGAL and WITH_CGAL_TESTS.

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

code/deps/CMakeLists.txt

@@ -5,9 +5,10 @@
 #
 # Which deps are extracted depends on the active build mode:
 #
-#   (default)      Eigen only          → tests-only build
+#   (default)           Eigen only
+#   WITH_CGAL_TESTS=ON  + Eigen, CGAL           (headless CI — no viewer)
 #   WITH_VIEWER=ON      + Eigen, libigl, libigl-glad, glfw
-#   WITH_CGAL=ON   + Eigen, CGAL       (WITH_VIEWER is implied by WITH_CGAL)
+#   WITH_CGAL=ON        + Eigen, CGAL, libigl, libigl-glad, glfw
 #
 
 function(setup_dependency NAME SUBDIR)
@@ -51,6 +52,7 @@ if(WITH_VIEWER)
 endif()
 
 # ── CGAL mode ─────────────────────────────────────────────────────────────────
-if(WITH_CGAL)
+# Both WITH_CGAL_TESTS (headless CI) and WITH_CGAL (full build) need CGAL headers.
+if(WITH_CGAL OR WITH_CGAL_TESTS)
     setup_dependency("CGAL-6.1.1" "include")
 endif()