feat: 实现存档界面与场景切换，优化文字渲染

- 新增 SavePanel/SaveSlot 存档选择界面，支持已有存档与新建存档
- 新增 GameState 单例与 MainGame 入口，按参数区分新游戏/读档
- 主菜单接入中文字体，返回/进入存档面板流程打通
- 开启 2D/3D MSAA、TAA 与屏幕空间 AA，消除放大文字锯齿
- 设置 1920x1080 视口与中文本地化过滤

src/Core/GameState.gd

@@ -0,0 +1,14 @@
+extends Node
+
+enum GameMode { NEW_GAME, LOAD_GAME }
+
+var mode: GameMode = GameMode.NEW_GAME
+var save_id: String = ""
+
+func request_new_game() -> void:
+	mode = GameMode.NEW_GAME
+	save_id = ""
+
+func request_load_game(id: String) -> void:
+	mode = GameMode.LOAD_GAME
+	save_id = id

src/Core/GameState.gd.uid

@@ -0,0 +1 @@
+uid://d1ajv537oir2l

src/Core/MainGame.gd

@@ -0,0 +1,9 @@
+extends Node
+
+func _ready() -> void:
+	pass
+
+
+func _on_top_bar_hud_sign_return_mainmenu() -> void:
+	get_tree().change_scene_to_file("res://src/UI/Panels/MainMenu/MainMenu.tscn")
+	pass # Replace with function body.

src/Core/MainGame.gd.uid

@@ -0,0 +1 @@
+uid://centrk8l54j7c

src/Core/MainGame.tscn

@@ -1,3 +1,11 @@
 [gd_scene format=3 uid="uid://box7tj5iyiopq"]
 
+[ext_resource type="Script" uid="uid://centrk8l54j7c" path="res://src/Core/MainGame.gd" id="1_8bu23"]
+[ext_resource type="PackedScene" uid="uid://dl0tb68vsw3c6" path="res://src/UI/HUD/TopBarHUD.tscn" id="1_g6xmk"]
+
 [node name="MainGame" type="Node" unique_id=2053561265]
+script = ExtResource("1_8bu23")
+
+[node name="TopBarHud" parent="." unique_id=1115394694 instance=ExtResource("1_g6xmk")]
+
+[connection signal="sign_return_mainmenu" from="TopBarHud" to="." method="_on_top_bar_hud_sign_return_mainmenu"]