Split Settings into model, serializer, paths and store

698 lines did all of it at once. The mutable CaptureFromUi callback becomes an
ISettingsCapture the settings view implements.

StarterNG.Tests/Fakes/FakeEnvironment.cs

@@ -0,0 +1,34 @@
+using StarterNG.Application.Abstractions;
+
+namespace StarterNG.Tests.Fakes;
+
+/// <summary>A host machine described inline: platform, home folder, env vars.</summary>
+public sealed class FakeEnvironment : IEnvironment
+{
+    private readonly Dictionary<string, string> _variables = new(StringComparer.Ordinal);
+
+    public bool IsWindows { get; set; }
+
+    public bool IsLinux { get; set; } = true;
+
+    public bool IsMacOS { get; set; }
+
+    public string BaseDirectory { get; set; } = Path.Combine("game");
+
+    public string? GetVariable(string name) => _variables.TryGetValue(name, out var value) ? value : null;
+
+    public FakeEnvironment With(string name, string value)
+    {
+        _variables[name] = value;
+        return this;
+    }
+
+    /// <summary>Switches the fake to Windows, with an APPDATA folder.</summary>
+    public FakeEnvironment AsWindows(string appData = "C:/Users/kolejarz/AppData/Roaming")
+    {
+        IsWindows = true;
+        IsLinux = false;
+        IsMacOS = false;
+        return With("APPDATA", appData);
+    }
+}