feat: 新增载具可堆叠规则（矿车/船堆叠至64）

mixin ItemInstance.getMaxStackSize，规则开启时矿车/船返回64，实现自动堆叠。新增item mixin分组。

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

@@ -77,4 +77,7 @@ public class SLSCarpetSettings {
 
     @Rule(categories = {SLSA, RuleCategory.FEATURE})
     public static boolean unbreakableDecoratedPot = false;
+
+    @Rule(categories = {SLSA, RuleCategory.FEATURE})
+    public static boolean stackableVehicles = false;
 }

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

@@ -0,0 +1,40 @@
+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 (!(this instanceof ItemStack stack)) {
+            return;
+        }
+        Item item = stack.getItem();
+        if (item instanceof MinecartItem || item instanceof BoatItem) {
+            cir.setReturnValue(64);
+        }
+    }
+}