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.

The issue is in the first-time setup menu logic in BurikoScriptFile.cs and MODAudioSet.cs. The global flag 'GAudioSet' is not saved unless the game exits cleanly, which is problematic on Steam Deck where force-closing is common. Start by examining how global flags are saved and loaded, particularly around game exit. Run the game on a Proton setup to reproduce the issue, and check if the save logic needs to be triggered earlier or more robustly.

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