tweak ctrl entity building to be bucketed per 'entity store' - we can now have multiple 'entity stores', one on the frontend thread, one on the ctrl thread, and keep them in sync at specific points via event lists

src/df/core/df_core.h

@@ -1179,6 +1179,7 @@ struct DF_State
   B32 ctrl_solo_stepping_mode;
   
   // rjf: control thread ctrl -> user reading state
+  CTRL_EntityStore *ctrl_entity_store;
   Arena *ctrl_stop_arena;
   CTRL_Event ctrl_last_stop_event;