fix: 修复objective重复创建崩溃（ensure推迟到scoreboard load后）

onServerLoaded时机太早，scoreboard.dat还没load完，getObjective返回null导致重复addObjective崩溃。改为syncScores首次调用时ensure。

src/main/java/com/github/zly2006/carpetslsaddition/ServerMain.java

@@ -54,7 +54,6 @@ public class ServerMain implements ModInitializer, CarpetExtension {
     public void onServerLoaded(MinecraftServer server) {
         ServerMain.server = server;
         com.github.zly2006.carpetslsaddition.util.BotListManager.load();
-        com.github.zly2006.carpetslsaddition.util.BoardManager.ensureObjectives(server);
         startBoardWatcher(server);
     }
 

src/main/java/com/github/zly2006/carpetslsaddition/util/BoardManager.java

@@ -74,9 +74,15 @@ public final class BoardManager {
         access.set(value);
     }
 
+    private static boolean objectivesEnsured = false;
+
     /** 从 stats 同步所有在线玩家的分数到 scoreboard（定期调用）。 */
     public static void syncScores(MinecraftServer server) {
         Scoreboard sb = server.getScoreboard();
+        if (!objectivesEnsured) {
+            ensureObjectives(server);
+            objectivesEnsured = true;
+        }
         Objective deaths = sb.getObjective(OBJ_DEATHS);
         Objective mined = sb.getObjective(OBJ_MINED);
         if (deaths == null && mined == null) {