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/Fakes/FixedClock.cs

@@ -0,0 +1,16 @@
+using StarterNG.Application.Abstractions;
+
+namespace StarterNG.Tests.Fakes;
+
+/// <summary>A clock frozen at a chosen instant.</summary>
+public sealed class FixedClock : IClock
+{
+    public FixedClock(DateTime now)
+    {
+        Now = now;
+    }
+
+    public DateTime Now { get; set; }
+
+    public DateTime UtcNow => Now.ToUniversalTime();
+}