Write settings, presets and favourites whole or not at all

A failed write leaves the old file in place. Settings that could not be read
are not saved over. UI exceptions reach the log. Started from a shortcut set
up elsewhere, the starter moves to its own folder when only that one holds a
game.

StarterNG.Tests/PhysicalFileSystemTests.cs

@@ -0,0 +1,36 @@
+using StarterNG.Infrastructure.Adapters;
+
+namespace StarterNG.Tests;
+
+public sealed class PhysicalFileSystemTests : IDisposable
+{
+    private readonly string _root = Directory.CreateTempSubdirectory("starterng-disk-").FullName;
+    private readonly PhysicalFileSystem _files = new();
+
+    public void Dispose() => Directory.Delete(_root, recursive: true);
+
+    [Fact]
+    public void An_atomic_write_makes_or_replaces_the_file_and_leaves_nothing_beside_it()
+    {
+        string path = Path.Combine(_root, "file.json");
+
+        _files.WriteAllTextAtomic(path, "first");
+        _files.WriteAllTextAtomic(path, "zażółć");
+
+        Assert.Equal("zażółć", File.ReadAllText(path));
+        Assert.Equal(new[] { path }, Directory.GetFiles(_root));
+    }
+
+    [Fact]
+    public void An_atomic_write_that_cannot_finish_leaves_what_stood_there()
+    {
+        string path = Path.Combine(_root, "file.json");
+        File.WriteAllText(path, "what stood there");
+        // Nowhere to put the new text on its way in.
+        Directory.CreateDirectory(path + PhysicalFileSystem.TempSuffix);
+
+        Assert.ThrowsAny<Exception>(() => _files.WriteAllTextAtomic(path, "half"));
+
+        Assert.Equal("what stood there", File.ReadAllText(path));
+    }
+}