Rib: accept a Project feature as its sketch ref

Rib guarded with `sk.type != CadFeatureType::Sketch`, while every other
sketch consumer — Extrude, SurfaceExtrude, SurfaceRevolve, the loft paths —
tests `!= Sketch && != Project`. A Project feature carries a plane and Line
entities, which is all a rib reads, so the guard blocked "project a body
edge, then rib along it" for no stated reason.

The picker in the Design tab offered Sketch features only, so it is widened
to match: a kernel that accepts Project refs and a GUI that never lists them
would have left the path unreachable anyway.

Worth recording for whoever hits this next: Rib also needs a sketch carrying
EXPLICIT entities. A parametric Rectangle sketch (add_sketch with
width/height) has an empty entities vector — build_sketch_wire synthesises
its profile on demand — so rib_entity 0 is out of range there and it fails
with "rib: bad entity". That is why Rib could not be driven headlessly at
all before this change; a Project feature is now the one programmatic way to
produce a ribbable line.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/slic3r/GUI/DesignPanel.cpp

@@ -596,7 +596,10 @@ DesignPanel::DesignPanel(wxWindow* parent)
                     for (int i = 0; i < int(m_doc.features.size()); ++i) {
                         // 3-arg Append: ComboBox's own Append(text, bitmap) hides
                         // wxItemContainer's (text, void*) — see the Sweep picker.
-                        if (m_doc.features[i].type == CadFeatureType::Sketch)
+                        // Project features too: they carry Line entities the kernel ribs from
+                        // just like a drawn sketch, so a projected body edge is a valid path.
+                        if (m_doc.features[i].type == CadFeatureType::Sketch ||
+                            m_doc.features[i].type == CadFeatureType::Project)
                             m_rib_sketch->Append(wxString::FromUTF8(m_doc.features[i].name), wxNullBitmap,
                                                  reinterpret_cast<void*>(intptr_t(i)));
                     }