Add a level 2 scaffold and make the level-clear screen load the next level

level_clear.gd gained a next_level_scene export: empty reloads the current level, otherwise it changes to the given scene. Level 1 now points at res://scenes/level_2.tscn. level_2.tscn is an empty but complete level: background, three TileMapLayers wired to the same TileSet, player, HUD, death screen and level-clear screen, plus the empty prefab containers. Its hidden_passages script explicitly sets hidden_cells = [] so level 1's passage cells are not carved here.

scripts/diamond.gd.uid

@@ -0,0 +1 @@
+uid://dbjrrn5xe6nbh

scripts/door.gd.uid

@@ -0,0 +1 @@
+uid://dr3fyoan8fwhj

scripts/level_clear.gd

@@ -6,6 +6,9 @@ extends CanvasLayer
 
 @export var fade_time: float = 0.8
 @export var button_delay: float = 0.3
+## 点「进入下一关」时加载哪个场景。
+## 留空 = 重新加载本关；填 res://scenes/level_2.tscn 之类就跳到下一关。
+@export_file("*.tscn") var next_level_scene: String = ""
 
 var _fade: ColorRect = null
 var _center: CenterContainer = null
@@ -29,6 +32,8 @@ func _ready() -> void:
 		_fade.color = Color(0.0, 0.0, 0.0, 0.0)
 	if _center != null:
 		_center.visible = false
+	if next_level_scene.is_empty() and _button != null:
+		_button.text = "重新开始  (R)"
 
 
 func show_clear() -> void:
@@ -65,8 +70,9 @@ func _unhandled_input(event: InputEvent) -> void:
 			next_level()
 
 
-## 目前项目里只有一关，所以"下一关"先做成重新加载本关。
-## 以后真加了第二关，把这里换成 change_scene_to_file("res://scenes/level2.tscn") 即可。
 func next_level() -> void:
 	get_tree().paused = false
-	get_tree().reload_current_scene()
+	if next_level_scene.is_empty():
+		get_tree().reload_current_scene()
+	else:
+		get_tree().change_scene_to_file(next_level_scene)

scripts/level_clear.gd.uid

@@ -0,0 +1 @@
+uid://cxfohn44nkm28