feat(子弹效果): 为蓝水晶子弹添加爆炸效果并调整相关配置

- 在BlueCrystal.gd中添加destroy方法创建爆炸效果
- 新增BlueCrystalExplosion.tscn爆炸效果场景
- 调整PurpleCrystalExplosion.tscn粒子数量
- 修复ItemDropped.gd中玩家实例有效性检查
- 将Rooster.tscn的武器更换为水晶系武器

scripts/Statemachine/ItemDropped.gd

@@ -41,7 +41,7 @@ func _physics_process(_delta):
 		refindPlayer()
 
 func canICollect():
-	return targetPlayer.inventoryMax[item] > targetPlayer.inventory[item]
+	return is_instance_valid(targetPlayer) && targetPlayer.inventoryMax[item] > targetPlayer.inventory[item]
 func collect():
 	collecting = true
 	animator.play("collect")
@@ -65,7 +65,10 @@ static func generate(
 		instance.add_to_group("drops")
 	return instance
 static func getDrops() -> Array[ItemDropped]:
-	return WorldManager.tree.get_nodes_in_group("drops") as Array[ItemDropped]
+	var result: Array[ItemDropped] = []
+	for drop in WorldManager.tree.get_nodes_in_group("drops"):
+		result.append(drop)
+	return result
 static func getDropsCanCollet() -> Array[ItemDropped]:
 	var result: Array[ItemDropped] = []
 	for drop in getDrops():