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/FakeStrings.cs

@@ -0,0 +1,26 @@
+using StarterNG.Application.Abstractions;
+
+namespace StarterNG.Tests.Fakes;
+
+/// <summary>
+/// Translation table that echoes keys back, so assertions can name the key that
+/// a screen or check would have shown.
+/// </summary>
+public sealed class FakeStrings : ILocalizedStrings
+{
+    private readonly Dictionary<string, string> _overrides = new(StringComparer.Ordinal);
+
+    public string this[string key] => _overrides.TryGetValue(key, out var value) ? value : key;
+
+    public string Opt(string key) => _overrides.TryGetValue(key, out var value) ? value : string.Empty;
+
+    public string CurrentLanguage => "English";
+
+    public string CurrentLangCode => "en";
+
+    public FakeStrings With(string key, string value)
+    {
+        _overrides[key] = value;
+        return this;
+    }
+}