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

CMakeLists.txt

@@ -119,6 +119,7 @@ endif()
 
 option(SLIC3R_STATIC            "Compile OrcaSlicer with static libraries (Boost, TBB)" ${SLIC3R_STATIC_INITIAL})
 option(SLIC3R_GUI               "Compile OrcaSlicer with GUI components (OpenGL, wxWidgets)" 1)
+option(SLIC3R_CAD               "Compile OrcaSlicer with the parametric Design/CAD tab (needs OCCT ModelingAlgorithms)" 1)
 option(SLIC3R_FHS               "Assume OrcaSlicer is to be installed in a FHS directory structure" 0)
 option(SLIC3R_PROFILE           "Compile OrcaSlicer with an invasive Shiny profiler" 0)
 option(SLIC3R_PCH               "Use precompiled headers" 1)
@@ -190,6 +191,10 @@ if (SLIC3R_GUI)
     add_definitions(-DSLIC3R_GUI)
 endif ()
 
+if (SLIC3R_CAD)
+    add_definitions(-DSLIC3R_CAD)
+endif ()
+
 if(SLIC3R_DESKTOP_INTEGRATION)
     add_definitions(-DSLIC3R_DESKTOP_INTEGRATION)
 endif ()