Unlinked global items of Serpulo and Erekir

core/src/mindustry/content/TechTree.java

@@ -93,6 +93,8 @@ public class TechTree{
         public Seq<Objective> objectives = new Seq<>();
         /** Nodes that depend on this node. */
         public final Seq<TechNode> children = new Seq<>();
+        /** Planet associated with this tech node. Null to auto-detect, or use Serpulo if no associated planet is found. */
+        public @Nullable Planet planet;
 
         public TechNode(@Nullable TechNode parent, UnlockableContent content, ItemStack[] requirements){
             if(parent != null){