CoordSys: pick a body first, x-ray the rest

In an assembly the face you want for a mate connector is nearly always behind
another body, and a click only ever returns the frontmost hit. Hiding the
occluder from the Parts list works but means leaving the tool mid-pick.

The CoordSys card now carries a Body chooser. Pick a body and every other one
drops to 0.25 alpha AND stops catching clicks, so the wanted face is both
visible and reachable in one gesture. "(all)" restores normal picking.

Deliberately NOT hit cycling: repeated-click cycling was removed from solid
picking as a charter L5/§10 violation (DesignSketchTool.cpp, "NO CYCLE"), and
re-introducing it here would make "click a face" a multi-click gesture again.

Mechanics: DesignSketchTool::set_pick_only_body() gates body_pickable(), which
every pick path already consults; DesignCanvas::set_xray_focus() drives both it
and the per-body alpha in reload(). The chooser stays a pick FILTER only --
coordsys_body still comes from the actual pick, so nothing in the kernel moves.

Body focus follows the CoordSys card: open_tool() reads it back from the combo
rather than clearing outright, because editing a CoordSys feature loads the card
(and its body) before open_tool runs.

snaporca-bgvk. NOT COMPILED: deps/build lacks OpenVDB so the GUI tree will not
configure here; reviewed by diff only.

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

src/slic3r/GUI/DesignPanel.hpp

@@ -112,6 +112,7 @@ private:
     void arm_coordsys_pick(CoordSysPick target);
     void apply_coordsys_refs(CadFeature& f) const;
     void refresh_coordsys_labels();
+    void refresh_cs_body_choice();   // fill the CoordSys body chooser from current document
     void reset_coordsys_refs();
     void on_add_surface_extrude();
     void on_add_surface_revolve();
@@ -721,6 +722,7 @@ private:
 
     // CoordSys controls (datum coordinate system: point + orthonormal frame).
     ComboBox*         m_coordsys_type{nullptr};      // CoordSysType: PointWorld/FaceAndDirection
+    ComboBox*         m_cs_body{nullptr};             // body-focus chooser: restrict picking to one body
     wxSpinCtrlDouble* m_cs_x{nullptr};               wxSpinCtrlDouble* m_cs_y{nullptr};               wxSpinCtrlDouble* m_cs_z{nullptr};
     wxButton*         m_cs_pick_face{nullptr};       wxStaticText* m_cs_face_lbl{nullptr};
     wxButton*         m_cs_pick_edge{nullptr};       wxStaticText* m_cs_edge_lbl{nullptr};