feat(phase3a): introduce CGAL Surface_mesh as ConformalMesh foundation

Replaces the Java CoHDS with CGAL::Surface_mesh<Point3> (Simple_cartesian
kernel). Adds domain-specific property maps for lambda/theta/idx/alpha and
face geometry type — the direct C++ equivalent of CoVertex/CoEdge adapters.

New files:
  include/conformal_mesh.hpp   — ConformalMesh type + property-map helpers
  include/mesh_builder.hpp     — mesh factories (triangle, tetrahedron,
                                 quad-strip, fan) for tests and examples
  tests/cgal/                  — second test executable (conformallab_cgal_tests)
                                 built only with -DWITH_CGAL=ON

Test results (local, -DWITH_CGAL=ON):
  conformallab_tests:      36 registered | 23 passed | 13 skipped | 0 failed
  conformallab_cgal_tests: 14 registered | 14 passed |  0 skipped | 0 failed

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

code/tests/CMakeLists.txt

@@ -27,3 +27,8 @@ 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)
+    add_subdirectory(cgal)
+endif()