Look at each source file of a scenery once

The watch of scenery files asked three times per file, whether it is there,
its time and its size, every two seconds for every source of every scenery.
One look gives all three.

StarterNG.Tests/Fakes/InMemoryFileSystem.cs

@@ -200,6 +200,9 @@ public sealed class InMemoryFileSystem : IFileSystem
     /// <summary>Files added through <see cref="WithExecutable"/> carry the execute bit.</summary>
     public long GetFileSize(string path) => Bytes(path).LongLength;
 
+    public (DateTime LastWriteUtc, long Size)? LastWriteAndSize(string path) =>
+        FileExists(path) ? (GetLastWriteTimeUtc(path), GetFileSize(path)) : null;
+
     public bool IsExecutable(string path) => _executable.Contains(Normalize(path));
 
     public InMemoryFileSystem WithExecutable(string path, string contents = "\u007fELF")