Design tab: Mate tool + interference report — the 16 tools are now reachable

Mate completes the set: every CadDocument feature type now has a card. It is the
one that needed CoordSys wired first, since a mate connector IS a CoordSys
feature and cs_a/cs_b are feature indices into the recipe.

The card states what each kind constrains rather than just naming it, because
the five kinds are not distinguishable from their labels — the useful fact is
which DOF each PRESERVES: Fastened fixes all six, Planar leaves in-plane
sliding, Revolute leaves spin, Slider leaves axial travel, Cylindrical leaves
both. The offset/angle spins retitle per kind, since offset is a plane distance
for Planar and a position along the axis for the three joint kinds.

Confirm is blocked, with the reason in the status line, when fewer than two
CoordSys features exist or when A and B are the same one: a mate with cs_a ==
cs_b is meaningless and a dangling index recomputes to nothing useful.

check_interference() gets a button in the feature-tree header behind a rule, not
a tool card — it adds no feature, so it must not checkpoint(), recompute(), or
touch the undo stack, and a card would imply it does. Results go to the status
line as count + worst volume, with the per-pair list in a message box, named as
the parts tree names them.

Fixes on top of the generated wiring:
- the button's sizer adds sat after the closing brace of the block declaring
  trow, so trow was out of scope ("'trow' was not declared in this scope");
- the CoordSys client data was typed const void*, which Append rejects;
- the report labelled bodies 0-based while all 28 other body labels in this
  panel (and the parts tree) are 1-based, so it would have called the tree's
  "Body 2" an interference on "Body 1" — and it was the only unlocalised label.

Compiles clean (0 errors); kernel suite unchanged at 139 cases / 1960
assertions. Still to come: the M6 variables panel. The GUI has not yet been
exercised on a display.

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

src/slic3r/GUI/DesignPanel.hpp

@@ -51,7 +51,7 @@ public:
     void         mcp_after_change() { after_tree_edit(true); } // refresh tree + viewport + status
 
 private:
-    enum class Tool { None, Sketch, Extrude, Dressup, Hole, Thread, Shell, Revolve, Sweep, Pattern, Plane, Loft, Draft, Boolean, Cut, Insert, Axis, CoordSys, SurfaceExtrude, SurfaceRevolve, SurfaceLoft, SurfaceFill, SurfaceOffset, ThickenSurface, Transform, Mirror, Thicken, Rib, Project, DeleteFace, Helix };
+    enum class Tool { None, Sketch, Extrude, Dressup, Hole, Thread, Shell, Revolve, Sweep, Pattern, Plane, Loft, Draft, Boolean, Cut, Insert, Axis, CoordSys, SurfaceExtrude, SurfaceRevolve, SurfaceLoft, SurfaceFill, SurfaceOffset, ThickenSurface, Transform, Mirror, Thicken, Rib, Project, DeleteFace, Helix, Mate };
     // Plane tool: which datum reference the next solid pick fills (declared early so the
     // method decls + card lambdas below can name it).
     enum class PlanePick { None, FaceA, FaceB, EdgeA, EdgeB };
@@ -112,6 +112,8 @@ private:
     void on_add_project();
     void on_add_delete_face();
     void on_add_helix();
+    void on_add_mate();
+    void on_check_interference();
     // Fill m_bool_target / m_bool_tool / m_cut_target. as_of_feature < 0 = current bodies (add);
     // >= 0 = the bodies as they existed just before that feature index (Boolean re-edit, so a
     // consumed tool body still appears and its saved selection round-trips).
@@ -280,6 +282,7 @@ private:
     wxSizer*  m_box_project{nullptr};
     wxSizer*  m_box_delete_face{nullptr};
     wxSizer*  m_box_helix{nullptr};
+    wxSizer*  m_box_mate{nullptr};
     wxSizer*  m_box_insert{nullptr};   // Confirm/Cancel card for placing Text/SVG art
     int       m_insert_feat{-1};       // provisional imported-art feature awaiting Confirm
     // Move-body gizmo runs through the unified action bar too: Confirm keeps the placement,
@@ -324,6 +327,7 @@ private:
     wxStaticText* m_hdr_project{nullptr};
     wxStaticText* m_hdr_delete_face{nullptr};
     wxStaticText* m_hdr_helix{nullptr};
+    wxStaticText* m_hdr_mate{nullptr};
     wxStaticText* m_hdr_insert{nullptr};
 
     wxScrolledWindow* m_form{nullptr};
@@ -478,6 +482,19 @@ private:
     CheckBox*         m_helix_left_handed{nullptr};
     wxSpinCtrlDouble* m_helix_taper{nullptr};
 
+    // Mate (assembly) controls
+    ComboBox*         m_mate_kind{nullptr};
+    ComboBox*         m_mate_cs_a{nullptr};
+    ComboBox*         m_mate_cs_b{nullptr};
+    wxSpinCtrlDouble* m_mate_offset{nullptr};
+    wxSpinCtrlDouble* m_mate_angle{nullptr};
+    CheckBox*         m_mate_flip{nullptr};
+    wxStaticText*     m_offset_label{nullptr};
+    wxStaticText*     m_angle_label{nullptr};
+
+    // Feature-tree button
+    ScalableButton*   m_btn_interfere{nullptr};
+
     // Pattern controls (replicate the target body: linear or circular).
     ComboBox*         m_pattern_type{nullptr};      // 0 = Linear, 1 = Circular
     wxSpinCtrlDouble* m_pattern_count{nullptr};     // total instances incl. seed