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/CadDocument.hpp

@@ -19,6 +19,7 @@ namespace Slic3r {
 
 enum class CadFeatureType { Sketch, Extrude, Fillet, Chamfer, Hole, Thread, Shell, Revolve, Sweep, Pattern, Plane, Loft, Draft, Import, Boolean, Cut };
 enum class SketchShape    { Rectangle, Circle };
+enum class PlaneType      { Offset, Angle, Midplane, Tangent, TwoEdges, Coincident };
 enum class BooleanMode    { New, Add, Cut, Intersect };
 
 enum class ExtrudeEnd { Blind, Symmetric, TwoSided, ThroughAll, UpToFace, UpToVertex };
@@ -169,6 +170,17 @@ struct CadFeature {
     double      plane_offset{20};
     double      plane_angle_tilt{0};       // degrees (named *_tilt to avoid clash w/ revolve)
     int         plane_axis{0};             // tilt axis: 0 = base X, 1 = base Y
+    PlaneType   plane_type{PlaneType::Offset};
+    int         plane_face_body{-1};
+    int         plane_face{-1};
+    int         plane_face2_body{-1};
+    int         plane_face2{-1};
+    int         plane_edge_body{-1};
+    int         plane_edge{-1};
+    int         plane_edge2_body{-1};
+    int         plane_edge2{-1};
+    double      plane_u_size{60};
+    double      plane_v_size{60};
 
     // Boolean: combine two EXISTING bodies. `mode` reuses BooleanMode (Add = union,
     // Cut = subtract tool from target, Intersect = keep overlap; New unused). `target_body`
@@ -208,8 +220,10 @@ struct CadFeature {
            pattern_circular, pattern_count, pattern_spacing, pattern_dir, pattern_angle,
            plane_base, plane_offset, plane_angle_tilt, plane_axis,
            bool_tool_body, bool_keep_tool, bool_tolerance, bool_target_face, bool_tool_face,
-           cut_offset, cut_flip, cut_keep_upper, cut_keep_lower,
-           brep);
+            cut_offset, cut_flip, cut_keep_upper, cut_keep_lower,
+            brep,
+            plane_type, plane_face_body, plane_face, plane_face2_body, plane_face2,
+            plane_edge_body, plane_edge, plane_edge2_body, plane_edge2, plane_u_size, plane_v_size);
     }
     template<class Archive>
     void load(Archive& ar) {
@@ -230,7 +244,9 @@ struct CadFeature {
            plane_base, plane_offset, plane_angle_tilt, plane_axis,
            bool_tool_body, bool_keep_tool, bool_tolerance, bool_target_face, bool_tool_face,
            cut_offset, cut_flip, cut_keep_upper, cut_keep_lower,
-           brep);
+           brep,
+           plane_type, plane_face_body, plane_face, plane_face2_body, plane_face2,
+           plane_edge_body, plane_edge, plane_edge2_body, plane_edge2, plane_u_size, plane_v_size);
         imported_solid = brep_from_string(brep);
     }
 };
@@ -263,6 +279,11 @@ public:
     std::vector<int>        display_tri_body;  // per-triangle source body index (into bodies)
     std::string             error;             // last recompute error ("" = ok)
 
+    // Modeling origin: the world point the default XY/XZ/YZ planes pass through. The GUI sets this
+    // to the bed centre so sketches/datums land in the middle of the bed (not the bed corner =
+    // world 0). Not serialized — the GUI re-applies it from the live bed on every tab show.
+    Vec3d modeling_origin{Vec3d::Zero()};
+
     double linear_deflection{0.01};
     double angular_deflection{0.5};