Design: slot Radius caption, keyboard offer, plane combo removal, mass props, docs

Mirror of snaporca ac85277bac..0e7cb3ec78 (six changes, applied as a patch to
DesignPanel.cpp rather than copied, so this fork's 30 permitted divergent lines survive
— parity re-checked afterwards: the five shared files are byte-identical, DesignCanvas.cpp
and DesignPanel.cpp differ by exactly 16 and 30 lines).

- The straight slot's inline field says Radius, which is what it sets. It stores the
  half-width and passed the typed number through unchanged, so 30 produced a 60 mm slot.
- The offer opens from the keyboard (Menu, Shift+F10), anchored on the viewport rather
  than wherever the pointer happens to be. The card hint names the new route.
- The sketch card's Plane combo is gone; the plane comes from the viewport. Also stops
  build_candidate collapsing a face plane to a base plane while editing.
- Mass properties and the dead Edit row are wired into the offer; DesignOffer.hpp is
  regenerated from tool_atlas.json, verified by re-running the generator and diffing.
- docs/rig_build_traps.md + scripts/rig-build.sh, which derives its fork identity from
  project() so it cannot be pointed at the other fork's image or volume.
- docs/design_tab.md refreshed (44 commits stale) + a PR description, with this fork's
  own merge-base and diff shape rather than snaporca's.

Built green in the deps container with the new script and verified on the rig: Menu and
Shift+F10 both open the offer at the viewport centre with the pointer parked off-canvas,
and the sketch card shows no Plane row.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LyRwbuq6fjn3VV9U9UvhBM

src/slic3r/GUI/DesignPanel.hpp

@@ -127,6 +127,7 @@ private:
     void on_add_helix();
     void on_add_mate();
     void on_check_interference();
+    void on_mass_properties();          // read-only report on the selected solid; edits nothing
     // 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).
@@ -405,7 +406,6 @@ private:
     // from "nobody has chosen anything yet". This does.
     bool              m_plane_picked{false};
     ComboBox*         m_shape{nullptr};
-    ComboBox*         m_plane{nullptr};
     ComboBox*         m_mode{nullptr};
     wxSpinCtrlDouble* m_width{nullptr};
     wxSpinCtrlDouble* m_height{nullptr};
@@ -617,6 +617,11 @@ private:
     // the verbs that have no keyboard shortcut to route through.
     std::map<std::string, std::function<void()>> m_verb_actions;
     void show_offer_menu(const wxPoint& screen_pos);
+    // Where the offer opens when no mouse press anchors it: the keyboard route, and the automatic
+    // open on entering Sketch. The pointer if it is over the viewport, else the viewport's centre.
+    // A raw wxGetMousePosition() can be sitting on the toolbar, on the card column or on another
+    // monitor, and the menu would map there — detached from the geometry it is about.
+    wxPoint offer_anchor() const;
     int  offer_selection_kind() const;          // an OfferSel, as int to keep the header light
     // Does the SKETCH half of the map apply? A mode question, not a session one: begin_sketch
     // does not run until the first tool is armed, so between "press Sketch" and "pick a tool"