refactor WiiCompiled.Setup into WiiCompiled.Setup.Windows and add WiiCompiled.Setup.Common

the idea behind this is C# code that is OS agnostic can go in WiiCompiled.Setup.Common to be shared by any OS specific code (eg: WiiCompiled.Setup.Windows and WiiCompiled.Setup.Linux).

Launcher/WiiCompiled.Setup.Common/JsonState.cs

@@ -0,0 +1,33 @@
+using System.Text.Json;
+
+namespace WiiCompiled.Setup.Common;
+
+/// <summary>Reads and atomically writes the small JSON state documents each installer keeps.</summary>
+public static class JsonState
+{
+    private static readonly JsonSerializerOptions ReadOptions = new() { PropertyNameCaseInsensitive = true };
+    private static readonly JsonSerializerOptions WriteOptions = new() { WriteIndented = true };
+
+    public static T? TryRead<T>(string path) where T : class
+    {
+        try
+        {
+            return File.Exists(path) ? JsonSerializer.Deserialize<T>(File.ReadAllText(path), ReadOptions) : null;
+        }
+        catch
+        {
+            // A truncated or hand-edited state document must degrade into "unknown", which every
+            // caller already treats as "assume stale and rebuild", not into a crash.
+            return null;
+        }
+    }
+
+    public static void Write<T>(string path, T value)
+    {
+        var directory = Path.GetDirectoryName(path);
+        if (!string.IsNullOrEmpty(directory)) Directory.CreateDirectory(directory);
+        var tempPath = path + ".tmp-" + Guid.NewGuid().ToString("N");
+        File.WriteAllText(tempPath, JsonSerializer.Serialize(value, WriteOptions));
+        File.Move(tempPath, path, overwrite: true);
+    }
+}