Import a triangle mesh as an editable B-rep body (mesh2step port)

Opening an STL/OBJ in the Design pane now rebuilds it into a real OCCT B-rep
solid that the face/edge feature tools can operate on, instead of a print mesh.

GeometryEngine::mesh_to_brep is a native C++ port of mesh2step
(github.com/tommasobbianchi/mesh2step): vertices and edges are shared across
triangles at construction time (vertex cache by deduped index, edge cache by
unordered index pair), so no BRepBuilderAPI_Sewing pass is needed to rebuild the
topology afterwards, and watertightness falls out of the edge-usage counts for
free. An open mesh is returned as a shell and reported as such — never dressed up
as a fake solid.

It runs in-process on the OCCT kernel libslic3r already links, so no STEP file is
written or re-read. That is not an optimisation but the whole point: a faceted
STEP of a 62k-triangle mesh is ~149 MB and OCCT's STEPControl_Reader takes >300 s
to parse it back, so routing this through a file would hang the GUI.

Coplanar neighbours are merged (ShapeUpgrade_UnifySameDomain, 5° default) so the
body arrives with pickable CAD faces rather than one face per triangle — on the
20,656-triangle test part that is 20,614 faces down to 4,784. Without it the
import is technically a solid but nothing you can meaningfully fillet or extrude.

- Design pane: "Import mesh" button + Shift+M; warns above 50k triangles.
- MCP: import_mesh {path, tolerance, merge_angle_deg}, returning the full
  conversion stats so a caller can tell an honest solid from an open shell.
- Catch2: cube round-trip (exact volume, 12 faceted faces, 6 after merge), open
  mesh stays a shell, and the scale-independent sliver rule that a naive
  area < tolerance^2 test would get wrong.

Verified end-to-end on the real 20,656-triangle ir3v2 hotend STL: reproduces
mesh2step's Python run exactly (20,614 kept, 42 degenerate, 0 boundary edges,
2 non-manifold edges, not watertight) and the resulting body's bbox matches the
one FreeCAD reports for the same part.

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

src/slic3r/GUI/DesignPanel.hpp

@@ -90,6 +90,7 @@ private:
     void on_add_text();
     void on_import_svg();
     void on_import_step();   // STEP -> editable B-rep body (keeps the OCCT solid, not a mesh)
+    void on_import_mesh();   // STL/OBJ -> B-rep body via GeometryEngine::mesh_to_brep
     bool place_on_face();    // Prepare's Place on Face (F): lay the selected body face on the bed
     void add_imported_sketch(const std::vector<std::vector<std::vector<Vec2d>>>& regions,
                              const wxString& base_name);