Stream negotiated canonical model reads with snapshot framing

internal/platform/canvas_model_http.go

@@ -172,6 +172,11 @@ func (s *service) getCanvasModel(w http.ResponseWriter, r *http.Request, project
 	if err = tx.Rollback(); err != nil {
 		return err
 	}
+	if r.Header.Get("Accept") == modelStreamType {
+		writeCanvasModelStream(r.Context(), w, model)
+		return nil
+	}
+	w.Header().Add("Vary", "Accept")
 	reply(w, 200, response{"model": model.view(nil)})
 	return nil
 }