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

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

CMakeLists.txt

@@ -241,6 +241,7 @@ add_executable(udpscope
   src/source/registry.cpp
   src/source/udps_source.cpp
   src/source/udps_kind.cpp
+  src/source/probe_source.cpp
   src/plot_host.cpp
   src/export.cpp
   src/search.cpp