Clear the NPC building owner set after the NPC load

After boot the NPC cache answers ownerIsNPC, so the set is not kept.

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

src/engine/server/world/WorldServer.java

@@ -535,6 +535,10 @@ public class WorldServer {
         DbManager.CityQueries.GET_ALL_CITIES();
         Logger.info("Loading all NPCs");
         DbManager.NPCQueries.GET_ALL_NPCS();
+
+        // NPCs are cached now; Building.runAfterLoad falls back to the NPC cache.
+        BuildingManager._npcBuildingOwners.clear();
+
         Logger.info("Loading all Mobiles");
         DbManager.MobQueries.GET_ALL_MOBS();