session file records how the source was reached

A session is named <host>_<port>.toml, which cannot say whether that source
was multicast or which interface it joined on, so a chooser row could not
reconnect to one. Both go in the top block: a bare key after a [[table]]
header belongs to that table, which is how the enabled key was lost once.

src/layout_store.cpp

@@ -242,6 +242,10 @@ bool load_layout(const std::string& path, LayoutState& out) {
           plot_count = std::max(0, std::stoi(value));
         } catch (...) {
         }
+      } else if (key == "multicast") {
+        st.multicast = strip_quote(value);
+      } else if (key == "interface") {
+        st.interface_addr = strip_quote(value);
       }
     } else if (section == Section::Plot && !st.rows.empty()) {
       if (key == "signals") {
@@ -325,6 +329,11 @@ bool save_layout(const std::string& path, const LayoutState& state) {
   o << "# udpscope session layout (written by the app; safe to edit)\n";
   if (state.window_s > 0.0) o << "window_s = " << state.window_s << "\n";
   o << "plots = " << state.rows.size() << "\n";
+  // Top level, before any table, for the same reason as `enabled` below.
+  if (!state.multicast.empty()) o << "multicast = " << toml_escape(state.multicast) << "\n";
+  if (!state.interface_addr.empty()) {
+    o << "interface = " << toml_escape(state.interface_addr) << "\n";
+  }
   // Top level, before any table. Emitted after the `[[calibration]]` tables it belonged to the last
   // of them -- a bare key after a header is that table's key in TOML -- so the parser, correctly,
   // never saw it and a session's enable state was silently lost.