plot host: rows hold 1..4 cells; split probe scaffold

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ui_app.hpp

@@ -221,6 +221,8 @@ struct App {
   /// a frame apart, because the buttons are placed from the rects the last frame was laid
   /// out with, so the sequence cannot be one function.
   int dbg_layout_step = 0;
+  /// Which step of the split probe runs next (`UDPSCOPE_PROBE_SPLIT_MS`).
+  int dbg_split_step = 0;
   /// 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`).
@@ -2095,6 +2097,9 @@ void layout_probe_report(App& a, const char* what);
 void layout_click(GtkWidget* b);
 void layout_probe_next(App& a, int step, guint delay_ms);
 gboolean layout_probe_step(gpointer user);
+void split_probe_report(App& a, const char* tag);
+void split_probe_next(App& a);
+gboolean split_probe_step(gpointer user);
 gboolean enable_all_probe(gpointer user);
 gboolean disarm_probe_step2(gpointer user);
 gboolean disarm_probe_step3(gpointer user);