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/CMakeLists.txt

@@ -7,24 +7,42 @@ message(STATUS "Configuring ${PROJECT_NAME}...")
 
 # ── Build modes ────────────────────────────────────────────────────────────────
 #
-#   Default (CI / tests-only): only Eigen + GTest are required.
+#   Default (CI fast / pure-math tests):
+#     cmake -S code -B build
+#     Only Eigen + GTest required.  36 non-CGAL tests.
 #
-#   -DWITH_CGAL=ON   builds the conformallab_core CLI app (needs CGAL).
-#                    Automatically enables WITH_VIEWER because the app uses
-#                    the viewer library for mesh visualisation.
+#   -DWITH_CGAL_TESTS=ON   CGAL test suite only — no viewer, no CLI app.
+#     cmake -S code -B build -DWITH_CGAL_TESTS=ON
+#     Requires: system Boost headers (apt install libboost-dev).
+#     Builds:   conformallab_cgal_tests  (158 tests).
+#     Does NOT require wayland-scanner, GLFW, libigl or a display.
+#     Use this in headless CI.
 #
-#   -DWITH_VIEWER=ON builds the viewer library standalone (libigl/GLFW/GLAD).
+#   -DWITH_CGAL=ON         Full build: CLI app + viewer + examples + CGAL tests.
+#     cmake -S code -B build -DWITH_CGAL=ON
+#     Requires: Boost + Wayland/X11 dev headers (wayland-scanner, libx11-dev …).
+#     Automatically enables WITH_VIEWER.
+#     Use this for local development with the interactive viewer.
+#
+#   -DWITH_VIEWER=ON        Viewer library only (libigl / GLFW / GLAD).
 #
 # ──────────────────────────────────────────────────────────────────────────────
-option(WITH_CGAL   "Build conformallab_core app (requires CGAL + Viewer)" OFF)
-option(WITH_VIEWER "Build viewer library (libigl / GLFW / GLAD)"          OFF)
+option(WITH_CGAL_TESTS "Build CGAL test suite without viewer/CLI (headless CI)" OFF)
+option(WITH_CGAL       "Build conformallab_core CLI app + viewer + CGAL tests"  OFF)
+option(WITH_VIEWER     "Build viewer library (libigl / GLFW / GLAD)"            OFF)
 
-# The CLI app always needs the viewer; enable it implicitly.
+# WITH_CGAL_TESTS is a strict subset of WITH_CGAL — no viewer, no CLI.
+# WITH_CGAL (full build) implies WITH_VIEWER.
 if(WITH_CGAL AND NOT WITH_VIEWER)
     message(STATUS "WITH_CGAL implies WITH_VIEWER – enabling automatically.")
     set(WITH_VIEWER ON CACHE BOOL "" FORCE)
 endif()
 
+# Propagate Boost requirement for both CGAL modes.
+if(WITH_CGAL OR WITH_CGAL_TESTS)
+    find_package(Boost REQUIRED)
+endif()
+
 # ── Standard settings ──────────────────────────────────────────────────────────
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
@@ -85,11 +103,6 @@ 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