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/player_bullet.gd

@@ -0,0 +1,37 @@
+class_name PlayerBullet
+extends Area2D
+## 玩家子弹：沿自身方向直线飞行（斜射弹共享此类），命中敌机造成伤害。
+
+const SPEED := 760.0
+const TEX := preload("res://assets/svg/bullet_player.svg")
+
+var direction := Vector2.UP
+
+
+func _ready() -> void:
+	collision_layer = 2
+	# 4 空中敌机 + 32 地面/海面单位（只被打，不与玩家机体碰撞）
+	collision_mask = 4 | 32
+	var sprite := Sprite2D.new()
+	sprite.texture = TEX
+	add_child(sprite)
+	var shape := CollisionShape2D.new()
+	var rect := RectangleShape2D.new()
+	rect.size = Vector2(6.0, 16.0)
+	shape.shape = rect
+	add_child(shape)
+	# 旋转节点本身：贴图与碰撞矩形一起对齐弹道方向
+	rotation = Vector2.UP.angle_to(direction)
+	area_entered.connect(_on_area_entered)
+
+
+func _process(delta: float) -> void:
+	position += direction * SPEED * delta
+	if global_position.y < -30.0 or global_position.x < -30.0 or global_position.x > 570.0:
+		queue_free()
+
+
+func _on_area_entered(area: Area2D) -> void:
+	if area is EnemyBase:
+		(area as EnemyBase).apply_damage(1)
+		queue_free()