refactor: what counts as an event moves into its own evaluator, so a search can ask the same question the trigger asks

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

CMakeLists.txt

@@ -105,6 +105,7 @@ add_executable(udpscope
   src/config.cpp
   src/channel.cpp
   src/model.cpp
+  src/condition.cpp
   src/trigger.cpp
   src/acquisition.cpp
   src/plot_host.cpp