Introduce layers, ports and a composition root

Ports in Application/Abstractions, adapters in Infrastructure, hand-wired
AppServices (no reflection, as PublishAot requires). Adds a test project.

StarterNG.Tests/GamePathsTests.cs

@@ -0,0 +1,26 @@
+using StarterNG.Infrastructure.Adapters;
+
+namespace StarterNG.Tests;
+
+public class GamePathsTests
+{
+    [Fact]
+    public void Wellknown_folders_hang_off_the_installation_root()
+    {
+        var paths = new GamePaths(Path.Combine("opt", "maszyna"));
+
+        Assert.Equal(Path.Combine("opt", "maszyna", "scenery"), paths.Scenery);
+        Assert.Equal(Path.Combine("opt", "maszyna", "dynamic"), paths.Dynamic);
+        Assert.Equal(Path.Combine("opt", "maszyna", "textures", "mini"), paths.MiniTextures);
+        Assert.Equal(Path.Combine("opt", "maszyna", "starter", "bledy.txt"), paths.DiagnosticsLog);
+    }
+
+    [Fact]
+    public void FromRoot_joins_segments_below_the_root()
+    {
+        var paths = new GamePaths("game");
+
+        Assert.Equal(Path.Combine("game", "data", "load_weights.txt"),
+                     paths.FromRoot("data", "load_weights.txt"));
+    }
+}