Design tab: reference planes at bed centre, slot dims, hole cube handle, real threads

Port of the snaporca CAD work to the mainline fork.

- Onshape default planes (XY/XZ/YZ) at the bed centre (transparent, labelled);
  modeling origin unified to the bed centre (CadDocument::modeling_origin);
  world-axis triad moved to the bed centre on the Design canvas only.
- Datum plane: clickable ghost-plane base pick + draggable offset arrow.
- Slot: dims reassessed to inter-centre distance / radius / angle; fixed the
  duplicate cap-arc radius quote.
- Hole: 3D cube move-handle on the face; binds to the face on the first click;
  decluttered side-distance construction lines.
- Thread: derive the M spec (diameter/pitch/depth) from a picked cylindrical
  surface or circular edge (GeometryEngine::circle_of_edge); fuse the helical
  ridge onto the existing body; MakePipeShell fixed-binormal sweep (uniform, no
  twist) + self-intersection/param guards.

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

src/slic3r/GUI/DesignCanvas.hpp

@@ -143,11 +143,22 @@ public:
                            double depth, double depth2, bool two_sided, bool flip);
     void clear_extrude_gizmo();
     void set_on_extrude_depth_changed(std::function<void(double, bool)> cb);
+    void set_datum_gizmo(const SketchPlane& plane, double usize, double vsize,
+                         const Vec3d& base_origin, const Vec3d& base_normal,
+                         double offset, bool offset_on);          // C3 resize handles + offset arrow
+    void clear_datum_gizmo();
+    void set_on_datum_size_changed(std::function<void(double, double)> cb);
+    void set_on_datum_offset_changed(std::function<void(double)> cb);
+    void set_base_pick(std::vector<SketchPlane> planes, std::vector<int> bases,
+                       std::vector<std::string> labels = {});   // clickable labelled reference planes
+    void clear_base_pick();
+    void set_on_datum_base_picked(std::function<void(int)> cb);
     void set_on_sketch_exit(std::function<void()> cb);           // Esc -> exit the tool
     void set_on_undo_redo(std::function<void(bool /*redo*/)> cb); // Ctrl+Z / Ctrl+Shift+Z
     // Persistently draw committed sketches (un-consumed ones stay visible).
     void set_display_sketches(std::vector<DesignSketchTool::DisplaySketch> ds);
-    void set_datum_planes(std::vector<SketchPlane> planes);   // draw datum/reference planes
+    void set_datum_planes(std::vector<SketchPlane> planes,
+                          std::vector<Vec2d> sizes = {});     // draw datum/reference planes (u/v extents)
     void set_body_highlight(bool on);   // tint the solid when its feature is tree-selected
     void set_body_translucent(bool on); // render the solid see-through (fillet/chamfer preview)
     void set_body_hidden(bool on);      // preview-only: hide base bodies, show only the result ghost
@@ -200,15 +211,16 @@ public:
     // Constraint glyph badges (C3.4b): the feature's constraints, drawn as iconic
     // marks near their entities in Constrain mode; empty clears them.
     void set_constraint_glyphs(std::vector<SketchEntityConstraintDef> cons);
-
-private:
-    void reload(bool keep_view);
     // Repaint the embedded canvas the right way for the active GL backend:
     // hardware GL gets a scheduled wxEVT_PAINT (render() runs inside the paint
     // cycle); software GL (llvmpipe etc.) gets a direct render() because a
     // scheduled Refresh() is frequently dropped there. Backend cached on first use.
+    // Public: DesignPanel calls it after a tree edit to force a frame on software GL.
     void request_repaint();
 
+private:
+    void reload(bool keep_view);
+
     wxGLCanvas* m_canvas_widget{nullptr};
     GLCanvas3D* m_canvas{nullptr};
     int         m_sw_gl{-1};   // -1 unknown, 0 hardware GL, 1 software GL