net8.0: upgrade from netcoreapp3.1, fix Linux rendering

- TFM: netcoreapp3.1 → net8.0, netstandard2.1 → net8.0
- System.Drawing.Common 5.0.3 → 6.0.0 (last version with Unix support)
- Remove stale packages (NETStandard.Library, System.Collections)
- Set System.Drawing.EnableUnixSupport via runtimeconfig + AppContext
- Pin System.Resources.Extensions 8.0.0, suppress NU1603
- Copy game_folder assets to build output (no run.sh required)
- Sta.CurrentDirectory → AppContext.BaseDirectory for portable path resolution
- Enable UnsafeBinaryFormatterSerialization + InvariantGlobalization
- Add global.json pinning SDK to 8.x
- Remove ambiguous `using System.Net.Mime.MediaTypeNames` in ParT.cs
- Add `using System.Reflection` to Program.cs

SlaveMatrix/SlaveMatrix/GameClasses/Program.cs

@@ -1,15 +1,19 @@
 using System;
+using System.Reflection;
 using _2DGAMELIB;
 
 namespace SlaveMatrix
 {
     internal static class Program
     {
+        static Program()
+        {
+            AppContext.SetSwitch("System.Drawing.EnableUnixSupport", true);
+        }
 
     	[STAThread]
     	private static void Main(string[] A_0)
     	{
-
             Sta.LoadConfig();
 
             double percent = 35.0;

SlaveMatrix/SlaveMatrix/GameClasses/Sta.cs

@@ -1410,7 +1410,7 @@ namespace SlaveMatrix
     		上着B_クロス初期化 = default(上着B_クロス情報);
     		上着B_前掛け初期化 = default(上着B_前掛け情報);
     		ブーツ初期化 = default(ブーツ情報);
-    		CurrentDirectory = Directory.GetCurrentDirectory();
+    		CurrentDirectory = AppContext.BaseDirectory;
     		GameData = new GameState();
     		SavePath = Path.Combine(CurrentDirectory, "save");
     		ImiPath = Path.Combine(CurrentDirectory, "text", "Basement", "Training", "Imitation.txt");