feat: /slsboard改发请求包，服务端给该玩家单独发display包（per-player显示）

客户端本地setDisplayObjective会被服务端覆盖，改用C2S请求包，服务端单独发ClientboundSetDisplayObjectivePacket给请求者。

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

@@ -0,0 +1,35 @@
+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;
+import net.minecraft.resources.Identifier;
+
+/**
+ * 客户端 → 服务端：请求显示某个榜单（或关闭）。
+ *
+ * <p>服务端收到后，给该玩家单独发送 {@code ClientboundSetDisplayObjectivePacket}，
+ * 实现"各玩家各自显示、互不影响"。
+ *
+ * <p>{@code objectiveName} 为空字符串表示关闭。
+ */
+public record BoardRequestPayload(String objectiveName) implements CustomPacketPayload {
+
+    public static final CustomPacketPayload.Type<BoardRequestPayload> TYPE =
+            new CustomPacketPayload.Type<>(Identifier.fromNamespaceAndPath("slsa", "board_request"));
+
+    public static final StreamCodec<RegistryFriendlyByteBuf, BoardRequestPayload> CODEC = StreamCodec.composite(
+            ByteBufCodecs.STRING_UTF8, BoardRequestPayload::objectiveName,
+            BoardRequestPayload::new
+    );
+
+    @Override
+    public CustomPacketPayload.Type<? extends CustomPacketPayload> type() {
+        return TYPE;
+    }
+
+    public static void register() {
+        net.fabricmc.fabric.api.networking.v1.PayloadTypeRegistry.serverboundPlay().register(TYPE, CODEC);
+    }
+}