fix(ci): decouple CGAL tests from viewer — add WITH_CGAL_TESTS flag

Root cause: -DWITH_CGAL=ON implied -DWITH_VIEWER=ON, which pulled in
GLFW, which requires wayland-scanner — not present in the headless CI
container (ubuntu:22.04 ARM64).

Fix: new CMake option -DWITH_CGAL_TESTS=ON builds conformallab_cgal_tests
without touching the viewer, GLFW, libigl, or any display dependency.
Only Boost headers are required (already in the Docker image).

Changes
───────
code/CMakeLists.txt
  - Add WITH_CGAL_TESTS option (OFF by default)
  - find_package(Boost REQUIRED) now shared between WITH_CGAL and WITH_CGAL_TESTS
  - WITH_CGAL still implies WITH_VIEWER (for local full builds)
  - Remove duplicate find_package(Boost) inside the WITH_CGAL block

code/tests/CMakeLists.txt
  - cgal/ subdirectory added for WITH_CGAL OR WITH_CGAL_TESTS

.gitea/workflows/cpp-tests.yml
  - test-cgal job: -DWITH_CGAL=ON → -DWITH_CGAL_TESTS=ON

README.md
  - Build modes table: three rows (default / CGAL_TESTS / CGAL full)
  - Quick-start: separate headless and full-local sections
  - Prerequisite table updated

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

code/tests/CMakeLists.txt

@@ -28,7 +28,9 @@ target_link_libraries(conformallab_tests PRIVATE GTest::gtest_main)
 include(GoogleTest)
 gtest_discover_tests(conformallab_tests DISCOVERY_TIMEOUT 60)
 
-# ── CGAL test suite (requires -DWITH_CGAL=ON) ────────────────────────────────
-if(WITH_CGAL)
+# ── CGAL test suite ──────────────────────────────────────────────────────────
+# Built with -DWITH_CGAL_TESTS=ON (headless CI, no viewer) or
+#            -DWITH_CGAL=ON       (full build with viewer + CLI).
+if(WITH_CGAL OR WITH_CGAL_TESTS)
     add_subdirectory(cgal)
 endif()