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/MainFrame.hpp

@@ -50,7 +50,9 @@ namespace GUI
 class Tab;
 class PrintHostQueueDialog;
 class Plater;
+#ifdef SLIC3R_CAD
 class DesignPanel;
+#endif
 class MainFrame;
 class ParamsDialog;
 #ifdef __WXGTK__
@@ -216,18 +218,22 @@ public:
     bool get_mac_full_screen() { return m_mac_fullscreen; }
 #endif
     //BBS GUI refactor
+    // Implicitly numbered: with SLIC3R_CAD off, tpDesign vanishes and every later tab
+    // falls back to the index it has upstream. Do not reintroduce explicit values.
     enum TabPosition
     {
         tpHome          = 0,
-        tpDesign        = 1,
-        tp3DEditor      = 2,
-        tpPreview       = 3,
-        tpMonitor       = 4,
-        tpMultiDevice   = 5,
-        tpProject       = 6,
-        tpCalibration   = 7,
-        tpAuxiliary     = 8,
-        toDebugTool     = 9,
+#ifdef SLIC3R_CAD
+        tpDesign,
+#endif
+        tp3DEditor,
+        tpPreview,
+        tpMonitor,
+        tpMultiDevice,
+        tpProject,
+        tpCalibration,
+        tpAuxiliary,
+        toDebugTool,
     };
 
     //BBS: add slice&&print status update logic
@@ -377,7 +383,9 @@ public:
     BBLTopbar*            m_topbar{ nullptr };
     PrintHostQueueDialog* printhost_queue_dlg() { return m_printhost_queue_dlg; }
     Plater*               m_plater { nullptr };
+#ifdef SLIC3R_CAD
     DesignPanel*          m_design_panel { nullptr };
+#endif
     //BBS: GUI refactor
     MonitorPanel*         m_monitor{ nullptr };