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/GLCanvas3D.cpp

@@ -7940,7 +7940,14 @@ void GLCanvas3D::_render_bed(const Transform3d& view_matrix, const Transform3d&
     */
     //bool show_texture = true;
     //BBS set axes mode
-    m_bed.set_axes_mode(m_main_toolbar.is_enabled());
+    if (m_axes_at_bed_center) {
+        // SnapOrca Design: triad at the bed centre = modeling origin (set every frame because
+        // set_shape/set_axes_mode otherwise reset it to the bed corner).
+        const Vec2d bc = m_bed.build_volume().bed_center();
+        m_bed.set_axes_origin(Vec3d(bc.x(), bc.y(), 0.0));
+    } else {
+        m_bed.set_axes_mode(m_main_toolbar.is_enabled());
+    }
     m_bed.render(*this, view_matrix, projection_matrix, bottom, scale_factor, show_axes);
 }