Document upright world planes; mod documentation

tests/StubTileRendererFactory.cs

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+namespace Cthangover.Mods.TileRender.Tests
+{
+    using System;
+    /// <summary>Registry fixture with no Godot allocation or dependency on an implementation mod.</summary>
+    internal class StubTileRendererFactory : ITileRendererFactory
+    {
+        public string Id { get; }
+        public StubTileRendererFactory(string id) { Id = id; }
+        public ITileProjection Projection(int size) => throw new NotSupportedException();
+        public ITileWorldPresentation Create() => throw new NotSupportedException();
+    }
+}