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

deps/OCCT/OCCT.cmake

@@ -4,6 +4,12 @@ else()
     set(library_build_type "Static")
 endif()
 
+# The parametric Design/CAD tab is the only consumer of OCCT's ModelingAlgorithms
+# module (fillet/offset/loft). With it OFF the deps prefix matches upstream exactly;
+# with it ON the delta is TKFillet + TKOffset (3.77 MiB, Windows only -- OCCT links
+# statically on macOS/Linux). Must match the SLIC3R_CAD passed to the main project.
+option(SLIC3R_CAD "Build OCCT's ModelingAlgorithms module (required by the Design/CAD tab)" ON)
+
 if (IN_GIT_REPO)
     set(OCCT_DIRECTORY_FLAG --directory ${BINARY_DIR_REL}/dep_OCCT-prefix/src/dep_OCCT)
 endif ()
@@ -28,7 +34,7 @@ orcaslicer_add_cmake_project(OCCT
         #-DBUILD_MODULE_DataExchange=OFF
         -DBUILD_MODULE_Draw=OFF
         -DBUILD_MODULE_FoundationClasses=OFF
-        -DBUILD_MODULE_ModelingAlgorithms=ON
+        -DBUILD_MODULE_ModelingAlgorithms=${SLIC3R_CAD}
         -DBUILD_MODULE_ModelingData=OFF
         -DBUILD_MODULE_Visualization=OFF
 )