Add -DSLIC3R_CAD compile-time gate for the Design/CAD tab

Gate the entire parametric Design/CAD subsystem behind a single CMake
option so the fork can be built with or without it. With SLIC3R_CAD OFF
the build is behaviour-neutral against upstream OrcaSlicer; this is the
"parallel build" for upstream integration discussion.

Gated surface:
- option(SLIC3R_CAD) + add_definitions(-DSLIC3R_CAD)
- deps/OCCT/OCCT.cmake: BUILD_MODULE_ModelingAlgorithms=${SLIC3R_CAD}
  (OFF matches upstream OCCT exactly; ON adds only TKFillet + TKOffset)
- TKFillet/TKOffset link, add_subdirectory(slvs) + libslvs
- CAD kernel + GUI sources, GLGizmoPrimitive (needs GeometryEngine),
  CAD Catch2 tests
- 11 upstream C++ hook sites (GLCanvas3D, MainFrame, GLGizmosManager)
- TabPosition and gizmo EType enums switched to implicit numbering so
  OFF reproduces upstream indices exactly

Verified on behemoth both ways: OFF and ON link snapmaker-orca +
libslic3r_tests (exit 0); ON shows the Design tab and passes 8 [design]
+ 1 [Deviation] tests, OFF omits them and shows upstream's tab layout.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BVzKmX6Y1aEteit1HTXG4Q

src/slic3r/GUI/Gizmos/GLGizmosManager.cpp

@@ -27,8 +27,10 @@
 #include "slic3r/GUI/Gizmos/GLGizmoSVG.hpp"
 #include "slic3r/GUI/Gizmos/GLGizmoMeshBoolean.hpp"
 #include "slic3r/GUI/Gizmos/GLGizmoAssembly.hpp"
+#ifdef SLIC3R_CAD
 #include "slic3r/GUI/Gizmos/GLGizmoPrimitive.hpp"
 #include "slic3r/GUI/Gizmos/GLGizmoSketch.hpp"
+#endif
 
 #include "libslic3r/format.hpp"
 #include "libslic3r/Model.hpp"
@@ -178,12 +180,14 @@ void GLGizmosManager::switch_gizmos_icon_filename()
         case (EType::BrimEars):
             gizmo->set_icon_filename(m_is_dark ? "toolbar_brimears_dark.svg" : "toolbar_brimears.svg");
             break;
+#ifdef SLIC3R_CAD
         case (EType::Primitive):
             gizmo->set_icon_filename(m_is_dark ? "toolbar_modifier_cube_dark.svg" : "toolbar_modifier_cube.svg");
             break;
         case (EType::Sketch):
             gizmo->set_icon_filename(m_is_dark ? "toolbar_sketch_dark.svg" : "toolbar_sketch.svg");
             break;
+#endif
         }
 
     }
@@ -227,8 +231,12 @@ bool GLGizmosManager::init()
     m_gizmos.emplace_back(new GLGizmoAssembly(m_parent, m_is_dark ? "toolbar_assembly_dark.svg" : "toolbar_assembly.svg", EType::Assembly));
     m_gizmos.emplace_back(new GLGizmoSimplify(m_parent, "reduce_triangles.svg", EType::Simplify));
     m_gizmos.emplace_back(new GLGizmoBrimEars(m_parent, m_is_dark ? "toolbar_brimears_dark.svg" : "toolbar_brimears.svg", EType::BrimEars));
+#ifdef SLIC3R_CAD
+    // Registered last: Primitive and Sketch are the final entries before Undefined, so
+    // omitting them leaves every preceding m_gizmos index (indexed by EType) untouched.
     m_gizmos.emplace_back(new GLGizmoPrimitive(m_parent, m_is_dark ? "toolbar_modifier_cube_dark.svg" : "toolbar_modifier_cube.svg", static_cast<unsigned int>(Primitive)));
     m_gizmos.emplace_back(new GLGizmoSketch(m_parent, m_is_dark ? "toolbar_sketch_dark.svg" : "toolbar_sketch.svg", static_cast<unsigned int>(Sketch)));
+#endif
     //m_gizmos.emplace_back(new GLGizmoSlaSupports(m_parent, "sla_supports.svg", sprite_id++));
     //m_gizmos.emplace_back(new GLGizmoFaceDetector(m_parent, "face recognition.svg", sprite_id++));
     //m_gizmos.emplace_back(new GLGizmoHollow(m_parent, "hollow.svg", sprite_id++));

src/slic3r/GUI/Gizmos/GLGizmosManager.hpp

@@ -90,8 +90,12 @@ public:
         Assembly,
         Simplify,
         BrimEars,
+#ifdef SLIC3R_CAD
+        // Both need the CAD kernel (GeometryEngine); keep them last so that with
+        // SLIC3R_CAD off the enum matches upstream's numbering exactly.
         Primitive,
         Sketch,
+#endif
         //SlaSupports,
         // BBS
         //FaceRecognition,