Orca-Cad: port SnapOrca Design (parametric CAD tab) onto mainline OrcaSlicer

Grafts the sketch-first CAD environment from snaporca-cad onto the mainline
OrcaSlicer/OrcaSlicer base (vs snaporca's Snapmaker/OrcaSlicer base):
- 133 new files: CadDocument/SketchEngine/GeometryEngine/SketchConstraints/
  SketchSolver/SketchInference/ThreadStandards + vendored libslvs solver;
  DesignPanel/DesignCanvas/DesignSketchTool/SketchInlineEditor GUI; GLGizmo
  Primitive/Sketch; 75 design icons; Catch2 tests.
- Integration hooks ported to mainline's diverged versions: Design tab in
  MainFrame, embedded design viewport + sketch overlay + per-canvas chrome
  suppression in GLCanvas3D/PartPlate, gizmo registration, Plater accessors,
  CMake wiring (libslvs subdir, CAD sources, OCCT ModelingAlgorithms=ON).

Structural integration complete; build verification pending.

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

docs/design_tab_upstream_portability.md

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+# Design (CAD) tab — upstream portability assessment
+
+**Question:** can the SnapOrca *Design tab* (sketch-first parametric CAD: sketch →
+constrain → extrude/revolve/fillet/hole/thread/shell, multi-body, undo) be contributed
+to **mainline OrcaSlicer** (2.4 dev) rather than living only in this Snapmaker fork?
+
+**Short answer: yes, technically clean — the feature is self-contained and vendor-neutral.
+The single real gatekeeper is whether upstream is willing to take on OpenCASCADE (OCCT) as a
+build dependency, i.e. whether OrcaSlicer wants to become a CAD-integrated slicer.**
+
+## Why it is portable
+
+- **Self-contained.** The feature is ~28 kLOC of *new* files (kernel
+  `CadDocument` / `SketchEngine` / `GeometryEngine` / `Sketch*`; GUI
+  `DesignPanel` / `DesignCanvas` / `DesignSketchTool` / `GLGizmoSketch`) plus a
+  vendored, self-contained SolveSpace solver (`src/libslic3r/slvs/`, ~10 kLOC, LGPL).
+- **Tiny, guarded injection into shared code (~2–3 % surface).** The only edits to
+  pre-existing OrcaSlicer files are: a `m_design_sketch_tool` member + a handful of
+  null-checked hooks in `GLCanvas3D` (render overlay, mouse routing, Delete/Esc/Ctrl+Z),
+  a tab member + construction in `MainFrame`, and a few forward declarations. **No
+  changes** to the slicing pipeline (Print/PrintObject/Layer/GCode), Plater, Tab, Model,
+  or the printer-profile/config system.
+- **Zero Snapmaker coupling.** No "Snapmaker" references in any Design/CAD file; icons
+  are generic `resources/images/design_*.svg`; the code is pure wxWidgets/OpenGL/OCCT.
+  It would build and run in any OrcaSlicer fork unchanged.
+- **Orthogonal git history.** The feature lives on `feature/cad-primitives` as a clean,
+  linear series of `Design:` / `C*` / `M*` commits with no interleaved fork-specific
+  work, so it cherry-picks onto a clean OrcaSlicer base without slicing-logic conflicts.
+
+## The real blocker: OCCT
+
+`src/libslic3r/CMakeLists.txt` links ~30 OCCT toolkits (`TKBRep TKFillet TKOffset
+TKBool TKPrim TKTopAlgo TKMath TKernel …`) via `find_package(OpenCASCADE REQUIRED)`.
+OCCT is large (hundreds of MB of binaries, +15–30 min to a clean deps build) and is a
+dependency **mainline OrcaSlicer has never carried**. Accepting it is an architectural
+decision about the project's scope, not a code problem.
+
+## Top work items to upstream (≈3–5 dev-days)
+
+1. **Make OCCT an *optional* dependency.** Add it to `deps/` like the other externals,
+   gate the whole feature behind a CMake option (`-DENABLE_DESIGN_CAD=OFF` by default),
+   and document OCCT install per-platform. Builds without OCCT simply omit the tab.
+2. **Guard the tab construction** in `MainFrame` (and the `GLCanvas3D` hooks) on that
+   same flag so a minimal build links and runs with no Design code at all.
+3. **Tests + docs + license hygiene.** Add regression coverage for the kernel
+   (sketch-solve, extrude, fillet, undo), ship the LGPL notice for the vendored
+   `slvs/`, and add user docs. (libslvs itself is self-contained, no external deps.)
+
+## Verdict
+
+Portability is **high (≈7/10): "needs moderate adaptation," not "deeply entangled."**
+The engineering to upstream is modest and mechanical (optional-dependency plumbing +
+tests/docs). The decision is strategic: **does OrcaSlicer want OCCT and a CAD tab?** If
+yes, this feature is a near-drop-in starting point.
+
+---
+*Generated 2026-06-21 from a read-only analysis of the `feature/cad-primitives` branch.
+Tracking issue: bd `snaporca-frp`.*