Design tab: M6 variables panel + per-feature expression bindings

M6 landed the kernel side as two plain public maps — CadDocument::variables and
CadFeature::expr — reachable only through MCP's set_variable / set_feature_expr.
Nothing in the GUI could create a variable, so the parametric layer was
unreachable from the Design tab.

Variables get a wxListCtrl (name, expression) with add/edit/remove below the
feature tree, since they are document-scope and must not live in a card that
only exists while a tool is open. Expression bindings get one generic row in the
feature-edit path — a field-name combo plus an expression box — rather than an
extra control on each of 32 cards.

Every mutation copies McpControl's sequence exactly, and the rollback is the
part that matters: checkpoint, mutate, recompute, undo() on failure. Without it
one typo leaves the recipe permanently unrecomputable, since load() replays the
whole list. Removing a variable a feature still references fails that recompute,
so it reports the reference rather than a bare evaluation error.

The field-name combo is deliberately editable: only 11 feature types get a
curated field list, and free text is what makes the other 21 reachable. That is
safe because assign_field() throws "unknown parameter: <name>" for anything it
does not know, inside recompute()'s try block — so a wrong name gives a clear
message and a rollback, never a silently dead binding.

Two fixes on top of the generated wiring:
- make_combo() passes wxCB_READONLY, under which Orca's ComboBox HIDES its text
  ctrl (ComboBox.cpp:51). There is no SetEditable() to undo that, so the field
  combo is constructed directly with style 0; that shows the ctrl with
  wxTE_PROCESS_ENTER and makes GetValue() return typed text.
- the field-list helper had been made a file-static function taking
  DesignPanel::Tool, which required moving Tool out of private and into the
  public API. It is now a private static member instead: 32 values of internal
  card state should not be published to satisfy a signature.

Compiles clean (0 errors); kernel suite unchanged at 139 cases / 1960
assertions. Phase B is complete on this fork — all 16 previously GUI-less tools
plus the variables panel. Not yet exercised on a display.

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

src/slic3r/GUI/DesignPanel.hpp

@@ -52,6 +52,10 @@ public:
 
 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, Mate };
+    // Which numeric fields an expression can be bound to, per feature type. A member rather
+    // than a file-static helper so Tool — 32 values of purely internal card state — does not
+    // have to become part of this panel's public API just to be named in a signature.
+    static std::vector<std::string> fields_for_tool(Tool t);
     // 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 };
@@ -284,6 +288,7 @@ private:
     wxSizer*  m_box_helix{nullptr};
     wxSizer*  m_box_mate{nullptr};
     wxSizer*  m_box_insert{nullptr};   // Confirm/Cancel card for placing Text/SVG art
+    wxSizer*  m_box_expr{nullptr};     // expression binding card (visible during edit only)
     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,
     // Cancel reverts to the pose captured when the move started.
@@ -492,6 +497,27 @@ private:
     wxStaticText*     m_offset_label{nullptr};
     wxStaticText*     m_angle_label{nullptr};
 
+    // Expression binding (per-feature, visible during edit only)
+    ComboBox*         m_expr_field{nullptr};     // field-name picker (editable)
+    wxTextCtrl*       m_expr_text{nullptr};      // expression string
+    wxButton*         m_expr_set_btn{nullptr};   // Apply / bind
+    wxButton*         m_expr_clear_btn{nullptr}; // Remove binding
+    wxStaticText*     m_expr_status{nullptr};    // shows current bindings for the edited feature
+    void              populate_expr_fields(Tool t);   // fill m_expr_field from feature-type fields
+    void              on_set_expr();                   // checkpoint + write -> recompute -> undo on fail
+    void              on_clear_expr();                 // remove selected binding
+
+    // Document variables panel (below the feature tree / parts)
+    StaticBox*        m_var_box{nullptr};
+    wxListCtrl*       m_var_list{nullptr};
+    wxButton*         m_btn_add_var{nullptr};
+    wxButton*         m_btn_edit_var{nullptr};
+    wxButton*         m_btn_del_var{nullptr};
+    void              refresh_variables();            // rebuild m_var_list from m_doc.variables
+    void              on_add_variable();
+    void              on_edit_variable();
+    void              on_remove_variable();
+
     // Feature-tree button
     ScalableButton*   m_btn_interfere{nullptr};