flatudp: the source, kind, mock sender and stream probes

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

src/ui.cpp

@@ -6121,6 +6121,9 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
   if (const char* env = g_getenv("UDPSCOPE_PROBE_FLATUDP")) {
     g_timeout_add(static_cast<guint>(std::atoi(env)), flatudp_probe, &app);
   }
+  if (const char* env = g_getenv("UDPSCOPE_PROBE_FLATUDP_GAP")) {
+    g_timeout_add(static_cast<guint>(std::atoi(env)), flatudp_probe_gap, &app);
+  }
 
   // Our own options were consumed by parse_args(); GTK would reject them, so only
   // the program name is handed to g_application_run().