save the connection with the session

collect_layout() now reports the multicast group and interface the app was
launched with, so a stored session says how to reach its source and not just
which host and port it was.

src/ui.cpp

@@ -266,6 +266,11 @@ struct App {
   /// CONFIG arrived.
   std::string source_host;
   std::uint16_t source_port = 0;
+  /// How that source was reached. Unused at runtime -- `Acquisition` has its own copy -- but
+  /// written into the session file, because `<host>_<port>.toml` can say neither, and the startup
+  /// chooser needs both to reconnect to a multicast source from a stored row.
+  std::string source_multicast;
+  std::string source_interface;
   guint layout_save_source = 0;
   bool layout_loaded = false;
   unsigned signal_generation = ~0u;
@@ -558,6 +563,9 @@ LayoutState collect_layout(App& app) {
   // The app's own lists are the pane's source of truth for both of these.
   st.math = app.math;
   st.measures = app.measures;
+  // The connection is the session's too: the file's name carries only host and port.
+  st.multicast = app.source_multicast;
+  st.interface_addr = app.source_interface;
   return st;
 }
 
@@ -7159,6 +7167,8 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
   // Which data source this session talks to: the persisted-layout file's name.
   app.source_host = cfg.server;
   app.source_port = cfg.port;
+  app.source_multicast = cfg.multicast;
+  app.source_interface = cfg.interface_addr;
 
   GtkApplication* application =
       // `NON_UNIQUE`: a scope is the kind of program a user runs twice side by side -- one