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/libslic3r/GeometryEngine.cpp

@@ -23,6 +23,7 @@
 #include <GProp_GProps.hxx>
 #include <GeomLProp_SLProps.hxx>
 #include <BRepAdaptor_Curve.hxx>
+#include <gp_Circ.hxx>
 #include <GCPnts_TangentialDeflection.hxx>
 #include <STEPControl_Reader.hxx>
 #include <IFSelect_ReturnStatus.hxx>
@@ -397,6 +398,23 @@ GeometryEngine::CylinderFace GeometryEngine::cylinder_of_face(const TopoDS_Face&
     return cf;
 }
 
+GeometryEngine::CylinderFace GeometryEngine::circle_of_edge(const TopoDS_Edge& edge)
+{
+    CylinderFace cf;
+    if (edge.IsNull()) return cf;
+    BRepAdaptor_Curve curve(edge);
+    if (curve.GetType() != GeomAbs_Circle) return cf;
+    const gp_Circ c  = curve.Circle();
+    const gp_Ax1  ax = c.Axis();
+    cf.base     = Vec3d(c.Location().X(), c.Location().Y(), c.Location().Z());
+    cf.axis     = Vec3d(ax.Direction().X(), ax.Direction().Y(), ax.Direction().Z());
+    cf.radius   = c.Radius();
+    cf.height   = 0.0;       // an edge carries no axial extent; the card keeps the current length
+    cf.internal = false;     // ambiguous from an edge alone — default external, user can toggle
+    cf.ok       = true;
+    return cf;
+}
+
 bool GeometryEngine::face_plane_bounds(const TopoDS_Face& face, const Vec3d& origin,
                                        const Vec3d& x_axis, const Vec3d& y_axis,
                                        double& umin, double& umax, double& vmin, double& vmax)