refactor: 重构主菜单 UI，将 StartMenu 替换为 MainMenu

- 删除旧 StartMenu.tscn，新建 MainMenu.tscn + MainMenu.gd
- 新增 MainGame.tscn 主游戏场景作为运行入口
- project.godot 配置 run/main_scene 启动场景
- 添加 doc/design/UI/StartMenu.md 主菜单设计文档
- 新增 assets/ 资源目录结构 (audio/fonts/sprites)
- 修复通用资源分析文档中的缩进问题

assets/sprites/ui/BG1.png.import

@@ -0,0 +1,40 @@
+[remap]
+
+importer="texture"
+type="CompressedTexture2D"
+uid="uid://caeqoc6wx8toy"
+path="res://.godot/imported/BG1.png-6bf4b9c36da6bbdf71bd2c8203f4cb53.ctex"
+metadata={
+"vram_texture": false
+}
+
+[deps]
+
+source_file="res://assets/sprites/ui/BG1.png"
+dest_files=["res://.godot/imported/BG1.png-6bf4b9c36da6bbdf71bd2c8203f4cb53.ctex"]
+
+[params]
+
+compress/mode=0
+compress/high_quality=false
+compress/lossy_quality=0.7
+compress/uastc_level=0
+compress/rdo_quality_loss=0.0
+compress/hdr_compression=1
+compress/normal_map=0
+compress/channel_pack=0
+mipmaps/generate=false
+mipmaps/limit=-1
+roughness/mode=0
+roughness/src_normal=""
+process/channel_remap/red=0
+process/channel_remap/green=1
+process/channel_remap/blue=2
+process/channel_remap/alpha=3
+process/fix_alpha_border=true
+process/premult_alpha=false
+process/normal_map_invert_y=false
+process/hdr_as_srgb=false
+process/hdr_clamp_exposure=false
+process/size_limit=0
+detect_3d/compress_to=1