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/DesignCanvas.cpp

@@ -1338,6 +1338,16 @@ bool DesignCanvas::is_constraining_entities() const
     return m_sketch_tool.is_constraining_entities();
 }
 
+int DesignCanvas::sketch_selection_count() const
+{
+    return int(m_sketch_tool.selection().size());
+}
+
+bool DesignCanvas::sketch_first_selected_type(SketchEntity::Type& out) const
+{
+    return m_sketch_tool.first_selected_type(out);
+}
+
 bool DesignCanvas::selected_constrain_entities(int& e0, int& e1) const
 {
     return m_sketch_tool.selected_constrain_entities(e0, e1);