fix: 载具堆叠改为Slot层，仅玩家背包可叠

删除全局ItemInstanceMixin（会让所有容器都叠）。新增SlotMixin：仅当container是玩家Inventory且物品是矿车/船时返回64。

src/main/java/com/github/zly2006/carpetslsaddition/mixin/item/ItemInstanceMixin.java

@@ -1,40 +0,0 @@
-package com.github.zly2006.carpetslsaddition.mixin.item;
-
-import com.github.zly2006.carpetslsaddition.SLSCarpetSettings;
-import net.minecraft.world.item.BoatItem;
-import net.minecraft.world.item.Item;
-import net.minecraft.world.item.ItemInstance;
-import net.minecraft.world.item.ItemStack;
-import net.minecraft.world.item.MinecartItem;
-import org.spongepowered.asm.mixin.Mixin;
-import org.spongepowered.asm.mixin.injection.At;
-import org.spongepowered.asm.mixin.injection.Inject;
-import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;
-
-/**
- * 载具堆叠：让矿车/船的物品可以堆叠到 64。
- *
- * <p>26.1.2 的堆叠上限来自 {@code ItemInstance.getMaxStackSize()} 的 default 实现：
- * 读取 {@code DataComponents.MAX_STACK_SIZE} 组件，默认 1（故矿车/船默认不堆叠）。
- *
- * <p>本 mixin 注入该 default 方法：规则 {@code stackableVehicles} 开启时，
- * 若物品是矿车（{@link MinecartItem}）或船（{@link BoatItem}），返回 64。
- * 由于堆叠上限变大，同类载具在拾取/整理时会自动合并，实现"自动堆叠"。
- */
-@Mixin(ItemInstance.class)
-public interface ItemInstanceMixin {
-
-    @Inject(method = "getMaxStackSize", at = @At("HEAD"), cancellable = true)
-    private void stackableVehicles(CallbackInfoReturnable<Integer> cir) {
-        if (!SLSCarpetSettings.stackableVehicles) {
-            return;
-        }
-        if (!((Object) this instanceof ItemStack stack)) {
-            return;
-        }
-        Item item = stack.getItem();
-        if (item instanceof MinecartItem || item instanceof BoatItem) {
-            cir.setReturnValue(64);
-        }
-    }
-}

src/main/java/com/github/zly2006/carpetslsaddition/mixin/item/SlotMixin.java

@@ -0,0 +1,45 @@
+package com.github.zly2006.carpetslsaddition.mixin.item;
+
+import com.github.zly2006.carpetslsaddition.SLSCarpetSettings;
+import net.minecraft.world.entity.player.Inventory;
+import net.minecraft.world.inventory.Slot;
+import net.minecraft.world.item.BoatItem;
+import net.minecraft.world.item.Item;
+import net.minecraft.world.item.ItemStack;
+import net.minecraft.world.item.MinecartItem;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;
+
+/**
+ * 载具堆叠（仅玩家背包）：矿车/船在【玩家背包】内可堆叠到 64，其他容器不堆叠。
+ *
+ * <p>原理：{@code Slot.getMaxStackSize(ItemStack)} = min(容器上限, 物品上限)。
+ * 载具的物品上限全局为 1（默认不叠）。本 mixin 在【容器是玩家背包 Inventory】且物品是载具时，
+ * 直接返回 64，绕过 min 限制。
+ *
+ * <p>这样只有玩家背包（物品栏 36 格）内能叠，箱子/漏斗/潜影盒等其他容器不受影响。
+ */
+@Mixin(Slot.class)
+public abstract class SlotMixin {
+
+    @Inject(method = "getMaxStackSize(Lnet/minecraft/world/item/ItemStack;)I", at = @At("HEAD"), cancellable = true)
+    private void stackableVehiclesInPlayerInventory(ItemStack stack, CallbackInfoReturnable<Integer> cir) {
+        if (!SLSCarpetSettings.stackableVehicles) {
+            return;
+        }
+        if (stack == null || stack.isEmpty()) {
+            return;
+        }
+        Slot self = (Slot) (Object) this;
+        // 仅玩家背包
+        if (!(self.container instanceof Inventory)) {
+            return;
+        }
+        Item item = stack.getItem();
+        if (item instanceof MinecartItem || item instanceof BoatItem) {
+            cir.setReturnValue(64);
+        }
+    }
+}