session: plots carry row and width; files without row load one cell per row

src/ui_app.hpp

@@ -223,6 +223,9 @@ struct App {
   int dbg_layout_step = 0;
   /// Which step of the split probe runs next (`UDPSCOPE_PROBE_SPLIT_MS`).
   int dbg_split_step = 0;
+  /// The split probe's round trip: the arrangement saved, and the legacy copy of its file.
+  std::string dbg_split_expect;
+  std::string dbg_split_legacy_path;
   /// Which step of the cursors probe runs next (`UDPSCOPE_PROBE_CURSORS_MS`).
   int dbg_cursors_step = 0;
   /// Which step of the fit probe runs next (`UDPSCOPE_PROBE_FIT_MS`).