Mate conflicts: mark the row that carries them, and name the way out

detect_mate_conflicts() has been filling m_doc.mate_conflicts on every recompute
since the kernel half landed, and nothing read it. The diagnostics existed and
were invisible — a conflicting assembly looked exactly like a working one.

The tree row is where they go, because the tree is where the user is already
looking for which feature to change. Three states, in precedence order:

  disabled  -> dim. A SUPPRESSED mate is the user's answer to a conflict, so it
               must read as suppressed rather than keep shouting about it.
  conflict  -> warn.
  otherwise -> normal.

Selecting a marked row puts the reason on the status line — "Mate3 already
positions Body 2", the cycle, the self-mate — and names the eye as the way to
suppress it. A message that describes a problem with no action is a message that
gets ignored; the action here is already one click away on the row just selected.

Deliberately NOT a modal, and deliberately not treated as a document error. The
document still evaluates with a conflict present: the mate graph merely has more
than one answer for a body, and which one wins is the thing the user needs to
see. Blocking the loop to say so would interrupt without helping.

The dimming of non-involved bodies from the original UX proposal is still not
implemented, on purpose: under transform composition a failure mid-chain
propagates, so "not involved" is not a well-defined set, and dimming the wrong
bodies would hide the context needed to understand the conflict.

Reviewed and compiled (libslic3r_gui, RC=0); not exercised. snaporca-bioq.

src/slic3r/GUI/DesignPanel.hpp

@@ -644,6 +644,9 @@ private:
     // Every status write goes through here so long hints wrap instead of clipping.
     void        set_status(const wxString& text);
     wxString    idle_hint() const;   // what to say when nothing is selected
+    // Reason detect_mate_conflicts() recorded for a feature, or nullptr. Marks the tree row and
+    // feeds the status line; a conflict is a diagnostic, not a document error.
+    const std::string* mate_conflict_reason(int feature) const;
 
     wxMenuItem* append_offer_item(wxMenu* menu, int id, const wxString& text,
                                   const struct OfferVerb& v);