Add CObjectTemplate marker interface for cache templates

ITEM, DEED and RUNE templates share one id space (JSON-Refactor-Plan D1).
Empty for now; the shared getters come with Task 1.1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/engine/parsers/objects/ItemTemplate.java

@@ -29,7 +29,7 @@ import java.util.LinkedHashMap;
 // Not to be confused with engine.objects.ItemTemplate, which is loaded
 // from the database. Read only once loaded.
 
-public class ItemTemplate {
+public class ItemTemplate implements CObjectTemplate {
 
     public static HashMap<Integer, ItemTemplate> _items = new HashMap<>();