CAD: right-click in a sketch offers verbs for what is selected, instead of for nothing

Select a line in a sketch, right-click, and every sketch verb was greyed: Trim,
Extend, Fillet, Chamfer, Offset, Mirror, the arrays, Constrain. The menu was
right and the selection was gone — two independent faults, each of which hid
the other.

FIRST, the Select-mode RightDown branch called clear_selection() before handing
the click back. Handing it back is correct: the m_right_consumed flag means "the
tool USED this right-click", and a plain right-click is not a gesture
terminator, so the offer should open. Clearing first is not: the offer describes
WHAT IS SELECTED, so wiping the selection guaranteed it could only ever describe
nothing. Deselection keeps its own gesture — left-click on empty space, a few
lines above in the same handler.

SECOND, offer_selection_kind() returned SkNone for every sketch state. The offer
table has always carried verbs for a selected line, arc, point or pair, but
nothing ever RETURNED those kinds, so fourteen rows were gated on selection bits
no code path could set. Classify the selection instead: SkLine / SkArc / SkPoint
/ Sk2Ent, via a first_selected_type() accessor on the tool and two forwarders on
the canvas.

Either fix alone measures as a failure — the classification is handed an empty
selection, or the preserved selection has no kind to match — which is why both
land together.

This is the second half of the report behind 3eb6e5d608: a user comparing the
Design tab with Onshape said "adding constraints seems to be missing"
(OrcaSlicer PR #15238). Constrain was one of the fourteen dead rows, and the
gesture that would have shown it threw the selection away first.

Verified at the machine on behemoth by Tommaso: select a line of a rectangle,
right-click, and the sketch verbs are live.

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

src/slic3r/GUI/DesignSketchTool.cpp

@@ -9432,11 +9432,17 @@ bool DesignSketchTool::on_mouse_impl(wxMouseEvent& evt, GLCanvas3D& canvas)
             return true;
         }
         if (evt.RightDown()) {
-            // Merely dropping a selection is not a gesture terminator: the m_right_consumed flag
-            // this return value feeds means "the tool USED this right-click", and suppressing the
-            // offer menu on a plain deselection would leave no way to reach the right-click menu
-            // again once any geometry exists. So clear, but hand the click back.
-            clear_selection();
+            // Hand the click back (return false) so the offer opens: the m_right_consumed flag
+            // this return value feeds means "the tool USED this right-click", and a plain
+            // right-click in Select mode is not a gesture terminator.
+            //
+            // But do NOT drop the selection on the way out. The offer menu describes WHAT IS
+            // SELECTED, so clearing first guaranteed it could only ever describe nothing: select
+            // a line, right-click, and the sketch verbs — Trim, Extend, Fillet, Chamfer, Offset,
+            // Mirror, the arrays, Constrain — were all greyed, because by the time the menu was
+            // built the line was no longer selected. Reported from the machine as "selected a
+            // line, right-click exit from selection: only create and reference are usable".
+            // Deselecting still has a gesture: left-click on empty space, a few lines above.
             return false;
         }
         return false;                               // let drag orbit the camera