Watanagashi on Steam Deck + Proton keeps asking for setup menu

## Initial Report

> I am playing Watanagashi on my steam deck via proton, for some reason it keeps giving me the first time setup menu whenever I launch it
>
> I ended up figuring out how to make it stop happening, I decided not to enable the opening when doing the setup and that seems to have made it stop happening
Specific settings I chose were
-Gin/ Hou bgm
-English
-Enable opening
>
> So I’m guessing it was other that specific combination causing the issue or just that last one

## Problem

I'm not sure if this relates the "Force closing the game doesn't save globals", which is worse on steam deck because it's easy to force close the game from the Steam UI. See https://github.com/07th-mod/higurashi-assembly/issues/136

## First Time Setup Menu Logic

The first time setup menu logic is here:

https://github.com/07th-mod/higurashi-assembly/blob/a2d6fe11a714b9b99073c4bae31dc938847c023e/Assets.Scripts.Core.Buriko/BurikoScriptFile.cs#L2796-L2802

https://github.com/07th-mod/higurashi-assembly/blob/a2d6fe11a714b9b99073c4bae31dc938847c023e/MOD.Scripts.Core.Audio/MODAudioSet.cs#L196-L210

Basically, if `GetGlobalFlag("GAudioSet")` returns `0`, we assume the audio set hasn't been set yet.

The setup menu also doesn't let you exit until it has been set to `1`, but that doesn't actually save the value - the value is only saved when you cleanly exit the game.

問題出在BurikoScriptFile.cs和MODAudioSet.cs中的首次設定選單邏輯上。全域標記'GAudioSet'只有在遊戲正常退出時才會儲存，這在強制關閉很常見的Steam Deck上是個問題。首先檢查全域標記是如何儲存和載入的，特別是圍繞遊戲退出時。在Proton設定上執行遊戲以重現問題，並檢查儲存邏輯是否需要更早或更穩健地觸發。

由索引模型根據 Issue 內容生成。