Add patrolling enemies and link level 1 to level 2

New walker prefab (mushroom_red sprite) patrols a platform, turns at walls and ledges, drains a heart on contact and dies when stomped, bouncing the player up. Three are placed in level2. The level 2 spawn point is set to cell (7,11) so the player starts standing on the floor instead of falling in from mid-air, and level 1's LevelClear now loads level2.tscn.

scripts/walker.gd

@@ -0,0 +1,87 @@
+extends CharacterBody2D
+
+## 小怪：在平台上左右巡逻，碰到玩家扣一颗心；
+## 被玩家从上方踩到就死，并且把玩家弹起来。
+## 走到平台边缘会自动掉头，所以不会掉下去。
+
+@export var speed: float = 28.0
+@export var damage: int = 1
+## 被踩死时把玩家往上弹的速度
+@export var stomp_bounce: float = -150.0
+## 巡逻方向：-1 向左，1 向右
+@export var start_direction: int = -1
+## 掉头后的冷却时间：防止贴着墙时每帧都掉头、结果原地抖动
+@export var turn_cooldown: float = 0.25
+
+var direction: int = -1
+var dead: bool = false
+
+var _turn_timer: float = 0.0
+
+@onready var sprite: Sprite2D = $Sprite
+@onready var floor_probe: RayCast2D = $FloorProbe
+@onready var touch_area: Area2D = $TouchArea
+
+
+func _ready() -> void:
+	add_to_group("enemy")
+	direction = -1 if start_direction < 0 else 1
+	_apply_direction()
+	touch_area.body_entered.connect(_on_touch)
+
+
+func _physics_process(delta: float) -> void:
+	if dead:
+		return
+
+	if _turn_timer > 0.0:
+		_turn_timer -= delta
+
+	if not is_on_floor():
+		velocity += get_gravity() * delta
+	velocity.x = float(direction) * speed
+
+	# 撞到墙 或 前方脚下没有地板 -> 掉头（带冷却，掉头后先走一段再说）
+	if _turn_timer <= 0.0:
+		if is_on_wall() or (is_on_floor() and not floor_probe.is_colliding()):
+			direction = -direction
+			_turn_timer = turn_cooldown
+			_apply_direction()
+
+	move_and_slide()
+
+
+func _apply_direction() -> void:
+	if sprite != null:
+		sprite.flip_h = direction > 0
+	if floor_probe != null:
+		floor_probe.position.x = float(direction) * 6.0
+
+
+func _on_touch(body: Node2D) -> void:
+	if dead or not body.has_method("hurt"):
+		return
+
+	# 玩家从上往下踩到 -> 小怪死，玩家弹一下
+	var p := body as CharacterBody2D
+	if p != null and p.velocity.y > 0.0 and p.global_position.y < global_position.y - 3.0:
+		p.velocity.y = stomp_bounce
+		_die()
+		return
+
+	body.hurt(damage)
+
+
+func _die() -> void:
+	dead = true
+	velocity = Vector2.ZERO
+	touch_area.set_deferred("monitoring", false)
+	set_deferred("collision_layer", 0)
+	set_deferred("collision_mask", 0)
+
+	# 被踩扁 + 淡出
+	var tw := create_tween()
+	tw.set_parallel(true)
+	tw.tween_property(sprite, "scale", Vector2(1.35, 0.2), 0.12)
+	tw.tween_property(sprite, "modulate:a", 0.0, 0.3)
+	tw.chain().tween_callback(queue_free)