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/powerup.gd

@@ -0,0 +1,40 @@
+class_name Powerup
+extends Area2D
+## 掉落道具：缓慢下落 + 左右摇摆 + 缓慢自转，玩家接触即收集。
+## kind: Kind.POWER 火力升级 / Kind.BOMB 炸弹+1
+
+enum Kind { POWER, BOMB }
+
+const FALL_SPEED := 105.0
+const SWAY_AMP := 26.0
+const TEX_POWER := preload("res://assets/svg/powerup.svg")
+const TEX_BOMB := preload("res://assets/svg/bomb_powerup.svg")
+
+var kind := Kind.POWER
+
+var _t := 0.0
+var _base_x := 0.0
+var _sprite: Sprite2D
+
+
+func _ready() -> void:
+	collision_layer = 16
+	collision_mask = 0
+	_base_x = position.x
+	_sprite = Sprite2D.new()
+	_sprite.texture = TEX_BOMB if kind == Kind.BOMB else TEX_POWER
+	add_child(_sprite)
+	var shape := CollisionShape2D.new()
+	var circle := CircleShape2D.new()
+	circle.radius = 15.0
+	shape.shape = circle
+	add_child(shape)
+
+
+func _process(delta: float) -> void:
+	_t += delta
+	position.y += FALL_SPEED * delta
+	position.x = _base_x + sin(_t * 1.9) * SWAY_AMP
+	_sprite.rotation += delta * 2.4
+	if position.y > 1010.0:
+		queue_free()