source: a synthetic probe kind, and sessions that remember their source kind

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

src/layout_store.hpp

@@ -8,6 +8,7 @@
 
 #include "math_channel.hpp"
 #include "measure.hpp"
+#include "source/source_config.hpp"
 
 namespace udpscope {
 
@@ -54,6 +55,12 @@ struct LayoutState {
   /// unicast; empty `interface_addr` is the default route.
   std::string multicast;
   std::string interface_addr;
+  /// Which source kind this session belongs to. Empty means `udps`: every file written before the
+  /// plugins existed. Only a kind other than `udps` is written, so a UDPS session file is
+  /// byte-identical to what earlier versions wrote.
+  std::string source_kind;
+  /// That kind's settings (`SourceConfig`), stored as top-level `source_opt_<key>` entries.
+  SourceConfig source_opts;
 
   /// The derived channels, graphs and all: the editor's work is part of the session, not of the
   /// process. A graph is stored as one string per node so the file stays in this store's TOML
@@ -144,6 +151,8 @@ struct KnownSource {
   std::uint16_t port = 0;
   std::string multicast;       ///< empty = unicast
   std::string interface_addr;  ///< empty = the default route
+  std::string kind = "udps";   ///< the session's source kind
+  SourceConfig opts;           ///< that kind's settings (empty for UDPS, whose fields are above)
   int rows = 0;                ///< plots the session had
   int signals = 0;             ///< named slots across those plots
   std::int64_t mtime = 0;      ///< orders the list; never shown