feat: 加slsBoardEnabled规则作榜单总开关（服务端规则控制客户端命令）

BoardTogglePayload同步开关状态；客户端/slsboard检查enabled，未开启则拒绝。ServerMain加轮询线程检测规则变化。

src/main/java/com/github/zly2006/carpetslsaddition/net/BoardTogglePayload.java

@@ -0,0 +1,31 @@
+package com.github.zly2006.carpetslsaddition.net;
+
+import net.minecraft.network.RegistryFriendlyByteBuf;
+import net.minecraft.network.codec.ByteBufCodecs;
+import net.minecraft.network.codec.StreamCodec;
+import net.minecraft.network.protocol.common.custom.CustomPacketPayload;
+
+/**
+ * 服务端 → 客户端：榜单功能总开关状态（对应 Carpet 规则 {@code slsBoardEnabled}）。
+ *
+ * <p>规则变化时下发；客户端收到后决定 {@code /slsboard} 是否可用。
+ */
+public record BoardTogglePayload(boolean enabled) implements CustomPacketPayload {
+
+    public static final CustomPacketPayload.Type<BoardTogglePayload> TYPE =
+            CustomPacketPayload.createType("slsa:board_toggle");
+
+    public static final StreamCodec<RegistryFriendlyByteBuf, BoardTogglePayload> CODEC = StreamCodec.composite(
+            ByteBufCodecs.BOOL, BoardTogglePayload::enabled,
+            BoardTogglePayload::new
+    );
+
+    @Override
+    public CustomPacketPayload.Type<? extends CustomPacketPayload> type() {
+        return TYPE;
+    }
+
+    public static void register() {
+        net.fabricmc.fabric.api.networking.v1.PayloadTypeRegistry.clientboundPlay().register(TYPE, CODEC);
+    }
+}