feat: 榜单客户端（接收包+/slsboard命令+右侧HUD）

BoardClient注册客户端入口：接收BoardSyncPayload、/slsboard deaths/mined/off切换显示、HudElementRegistry注册右侧HUD。各玩家本地状态独立，不同步。

src/main/java/com/github/zly2006/carpetslsaddition/client/BoardClient.java

@@ -0,0 +1,69 @@
+package com.github.zly2006.carpetslsaddition.client;
+
+import com.github.zly2006.carpetslsaddition.net.BoardSyncPayload;
+import com.mojang.brigadier.arguments.StringArgumentType;
+import net.fabricmc.api.ClientModInitializer;
+import net.fabricmc.api.EnvType;
+import net.fabricmc.api.Environment;
+import net.fabricmc.fabric.api.client.command.v2.ClientCommandRegistrationCallback;
+import net.fabricmc.fabric.api.client.command.v2.FabricClientCommandSource;
+import net.fabricmc.fabric.api.client.networking.v1.ClientPlayNetworking;
+import net.fabricmc.fabric.api.client.rendering.v1.hud.HudElementRegistry;
+import net.minecraft.network.chat.Component;
+import net.minecraft.resources.Identifier;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * 客户端入口：接收榜单数据、注册 /slsboard 命令、注册 HUD 渲染。
+ *
+ * <p>纯客户端功能：显示哪个榜由本地状态决定，各玩家可不同步。
+ */
+@Environment(EnvType.CLIENT)
+public class BoardClient implements ClientModInitializer {
+
+    public static final Identifier HUD_ID = Identifier.fromNamespaceAndPath("slsa", "board_hud");
+
+    @Override
+    public void onInitializeClient() {
+        // 1) 接收服务端下发的榜单数据
+        ClientPlayNetworking.registerGlobalReceiver(BoardSyncPayload.TYPE, (payload, context) -> {
+            List<BoardClientState.Entry> entries = new ArrayList<>();
+            for (BoardSyncPayload.Entry e : payload.entries()) {
+                entries.add(new BoardClientState.Entry(e.name(), e.score()));
+            }
+            context.client().execute(() -> BoardClientState.setBoard(payload.board(), entries));
+        });
+
+        // 2) /slsboard 命令（纯客户端，非 OP 也能用）
+        ClientCommandRegistrationCallback.EVENT.register((dispatcher, registryAccess) -> {
+            dispatcher.register(
+                    net.minecraft.commands.Commands.literal("slsboard")
+                            .then(net.minecraft.commands.Commands.literal("deaths")
+                                    .executes(c -> { BoardClientState.toggle("deaths"); feedback(c.getSource()); return 1; }))
+                            .then(net.minecraft.commands.Commands.literal("mined")
+                                    .executes(c -> { BoardClientState.toggle("mined"); feedback(c.getSource()); return 1; }))
+                            .then(net.minecraft.commands.Commands.literal("off")
+                                    .executes(c -> { BoardClientState.set(""); feedback(c.getSource()); return 1; }))
+                            .then(net.minecraft.commands.Commands.argument("board", StringArgumentType.word())
+                                    .suggests((c, b) -> net.minecraft.commands.SharedSuggestionProvider.suggest(
+                                            new String[]{"deaths", "mined", "off"}, b))
+                                    .executes(c -> {
+                                        String v = StringArgumentType.getString(c, "board");
+                                        BoardClientState.set("off".equals(v) ? "" : v);
+                                        feedback(c.getSource());
+                                        return 1;
+                                    }))
+            );
+        });
+
+        // 3) HUD 渲染
+        HudElementRegistry.addLast(HUD_ID, new BoardHudElement());
+    }
+
+    private static void feedback(FabricClientCommandSource source) {
+        String d = BoardClientState.displaying;
+        source.sendFeedback(Component.literal("[SLSA] 榜单显示：" + (d.isEmpty() ? "关闭" : d)));
+    }
+}