feat: 场景与核心玩法脚本（双机型/波次/Boss/道具/炸弹/地面单位/浮游炮激光）

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

scripts/enemy_bullet.gd

@@ -0,0 +1,26 @@
+class_name EnemyBullet
+extends Area2D
+## 敌方弹丸：直线飞行，出屏即回收。
+
+const TEX := preload("res://assets/svg/bullet_enemy.svg")
+
+var velocity := Vector2(0.0, 260.0)
+
+
+func _ready() -> void:
+	collision_layer = 8
+	collision_mask = 0
+	var sprite := Sprite2D.new()
+	sprite.texture = TEX
+	add_child(sprite)
+	var shape := CollisionShape2D.new()
+	var circle := CircleShape2D.new()
+	circle.radius = 5.0
+	shape.shape = circle
+	add_child(shape)
+
+
+func _process(delta: float) -> void:
+	position += velocity * delta
+	if position.y < -40.0 or position.y > 1000.0 or position.x < -40.0 or position.x > 580.0:
+		queue_free()