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

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

src/ui.cpp

@@ -6003,6 +6003,9 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
   if (const char* env = g_getenv("UDPSCOPE_PROBE_LAYOUT_MS")) {
     g_timeout_add(static_cast<guint>(std::atoi(env)), layout_probe_step, &app);
   }
+  if (const char* env = g_getenv("UDPSCOPE_PROBE_SPLIT_MS")) {
+    g_timeout_add(static_cast<guint>(std::atoi(env)), split_probe_step, &app);
+  }
   if (const char* env = g_getenv("UDPSCOPE_TRIG_TRACE_MS")) {
     g_timeout_add(static_cast<guint>(std::atoi(env)), trig_trace_probe, &app);
   }