Make Scenery a domain aggregate with no I/O

Parsing and loading move to Infrastructure/Sceneries. Weather becomes a value
object owning the "does this differ from the file" rule.

StarterNG/Application/Abstractions/IRandomSource.cs

@@ -0,0 +1,15 @@
+namespace StarterNG.Application.Abstractions;
+
+/// <summary>
+/// Port over randomness. Scenery weather deliberately varies between runs (fog
+/// distance, overcast presets, texture randomiser); routing it through a port
+/// keeps those rules assertable instead of flaky.
+/// </summary>
+public interface IRandomSource
+{
+    /// <summary>Inclusive lower bound, exclusive upper bound.</summary>
+    int Next(int minInclusive, int maxExclusive);
+
+    /// <summary>Uniform value in [0, 1).</summary>
+    double NextDouble();
+}

StarterNG/Application/Abstractions/ISceneryRepository.cs

@@ -0,0 +1,24 @@
+using System;
+using System.Collections.Generic;
+using StarterNG.Domain.Sceneries;
+
+namespace StarterNG.Application.Abstractions;
+
+/// <summary>How far a scenery sweep has got, for the splash screen.</summary>
+public readonly record struct SceneryLoadProgress(int Loaded, int Total, string FileName);
+
+/// <summary>
+/// Reads the scenarios of an installation. The only route from a .scn on disk to
+/// a <see cref="Scenery"/>; nothing else constructs one from a path.
+/// </summary>
+public interface ISceneryRepository
+{
+    /// <summary>
+    /// Every scenario in the installation, in no particular order. Files that
+    /// fail to parse are logged and skipped rather than failing the load.
+    /// </summary>
+    IReadOnlyList<Scenery> LoadAll(IProgress<SceneryLoadProgress>? progress = null);
+
+    /// <summary>Reloads a single scenario; null when it is gone or unreadable.</summary>
+    Scenery? Load(string path);
+}