fix debug engine incorrectly using visual run state to soft halt, when ctrl thread did not necessarily report it was running; was leading to 'phantom halts'; other small fixes, dead code elimination, and begin sketching out proper cross-window drag/drop

src/ui/ui_core.h

@@ -625,6 +625,9 @@ struct UI_State
   //- rjf: main arena
   Arena *arena;
   
+  //- rjf: fixed keys
+  UI_Key external_key;
+  
   //- rjf: build arenas
   Arena *build_arenas[2];
   U64 build_index;