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.cpp

@@ -37,8 +37,10 @@
 #include "I18N.hpp"
 #include "GLCanvas3D.hpp"
 #include "Plater.hpp"
+#ifdef SLIC3R_CAD
 #include "DesignPanel.hpp"
 #include "McpControl.hpp"
+#endif
 #include "WebViewDialog.hpp"
 #include "../Utils/Process.hpp"
 #include "format.hpp"
@@ -1016,8 +1018,10 @@ void MainFrame::update_layout()
     {
     case ESettingsLayout::Old:
     {
+#ifdef SLIC3R_CAD
         m_design_panel->Reparent(m_tabpanel);
         m_tabpanel->InsertPage(tpDesign, m_design_panel, _L("Design"), std::string("tab_design_active"), std::string("tab_design_active"), false);
+#endif
         m_plater->Reparent(m_tabpanel);
         m_tabpanel->InsertPage(tp3DEditor, m_plater, _L("Prepare"), std::string("tab_3d_active"), std::string("tab_3d_active"), false);
         m_tabpanel->InsertPage(tpPreview, m_plater, _L("Preview"), std::string("tab_preview_active"), std::string("tab_preview_active"), false);
@@ -1274,12 +1278,14 @@ void MainFrame::init_tabpanel() {
         }
         //else if (panel == m_param_panel)
         //    m_param_panel->OnActivate();
+#ifdef SLIC3R_CAD
         else if (panel == m_design_panel) {
             // Re-sync the Design bed to the active printer: the panel is built before the
             // printer profile is fully applied, so its bed must refresh on activation or the
             // grid (true bed) spills past the stale default bed quad.
             m_design_panel->on_tab_shown();
         }
+#endif
         else if (panel == m_monitor) {
             //monitor
         }
@@ -1329,8 +1335,10 @@ void MainFrame::init_tabpanel() {
     // Register the plater with the app BEFORE constructing DesignPanel: its
     // DesignCanvas reads wxGetApp().plater()->config() at construction time.
     wxGetApp().plater_ = m_plater;
+#ifdef SLIC3R_CAD
     m_design_panel = new DesignPanel(this);
     start_mcp_control_if_enabled();   // opens the MCP socket iff SNAPORCA_MCP is set
+#endif
     m_plater->SetBackgroundColour(*wxWHITE);
     m_plater->Hide();