Take the remaining I/O out of the domain and drop AppState.Instance

Scenery translations and load weights get repositories; AppState moves to the
application layer.

StarterNG/Application/Abstractions/ISceneryTranslations.cs

@@ -0,0 +1,23 @@
+using StarterNG.Domain.Sceneries;
+
+namespace StarterNG.Application.Abstractions;
+
+/// <summary>
+/// The translations a scenery ships alongside itself, in scenery/i18n/*.json.
+/// </summary>
+/// <remarks>
+/// Scenery text may be an "@key" reference rather than literal prose, resolved
+/// against the currently loaded scenery's table. The table is per scenery, so
+/// <see cref="LoadFor"/> replaces it rather than adding to it.
+/// </remarks>
+public interface ISceneryTranslations
+{
+    /// <summary>Loads the tables for one scenery, English first then the user's language.</summary>
+    void LoadFor(Scenery scenery, string langCode);
+
+    /// <summary>
+    /// Resolves an "@key" reference against the loaded table. Text that is not a
+    /// reference, or a key with no translation, comes back unchanged.
+    /// </summary>
+    string Translate(string? text);
+}