Handle Psi's ReaperSurface::Count sentinel in decideDropClass — Gamma's exhaustive-switch gate turns it into a hard error on contact

The sentinel is not a surface, so it breaks to the existing unclassifiable-surface refusal rather than joining a real case label.

src/core/ui/drag_out.cpp

@@ -44,6 +44,9 @@ DropClass decideDropClass(int px, int py, const PanelClientRect& client,
         case ReaperSurface::FxEmbed:
         case ReaperSurface::Other:
             return DropClass::Refuse;
+
+        case ReaperSurface::Count:
+            break;  // the sentinel is not a surface; it falls to the refusal below
     }
     return DropClass::Refuse;  // an unclassifiable surface still refuses visibly, never silently
 }