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.Tests/Fakes/StubRandom.cs

@@ -0,0 +1,22 @@
+using StarterNG.Application.Abstractions;
+
+namespace StarterNG.Tests.Fakes;
+
+/// <summary>
+/// Randomness pinned to the low end of every range, so weather that varies in
+/// production is deterministic in tests.
+/// </summary>
+public sealed class StubRandom : IRandomSource
+{
+    private readonly int _fixedValue;
+
+    public StubRandom(int fixedValue = 0)
+    {
+        _fixedValue = fixedValue;
+    }
+
+    public int Next(int minInclusive, int maxExclusive) =>
+        Math.Clamp(minInclusive + _fixedValue, minInclusive, Math.Max(minInclusive, maxExclusive - 1));
+
+    public double NextDouble() => 0.0;
+}