Document upright world planes; mod documentation

tests/TileRendererRegistryTests.cs

@@ -0,0 +1,31 @@
+namespace Cthangover.Mods.TileRender.Tests
+{
+    using Cthangover.Core.Mods.Testing;
+    /// <summary>Explicit selection, ambiguity and owner-sensitive unregister never depend on discovery order.</summary>
+    public class TileRendererRegistryTests
+    {
+        private readonly TileRendererRegistry registry = new();
+        private readonly StubTileRendererFactory square = new("square");
+        private void Missing() => registry.Resolve("missing");
+        private void Ambiguous() => registry.Resolve(null);
+        private void Duplicate() => registry.Register(square);
+
+        [ModTest("registry/selection_collision_and_unload")]
+        static void SelectionCollisionAndUnload(ModTestContext ctx)
+        {
+            var test = new TileRendererRegistryTests();
+            test.registry.Register(test.square);
+            ModAssert.True(ReferenceEquals(test.square, test.registry.Resolve(null)), "single implementation may be inferred");
+            ModAssert.Throws(test.Missing, "explicit missing ID must not fall back", true);
+            ModAssert.Throws(test.Duplicate, "duplicate ID is rejected", true);
+            var iso = new StubTileRendererFactory("iso");
+            test.registry.Register(iso);
+            ModAssert.Throws(test.Ambiguous, "multiple implementations require selection", true);
+            ModAssert.True(ReferenceEquals(iso, test.registry.Resolve("iso")), "explicit selection");
+            test.registry.Unregister(new StubTileRendererFactory("iso"));
+            ModAssert.True(ReferenceEquals(iso, test.registry.Resolve("iso")), "a different owner cannot delete a factory");
+            test.registry.Unregister(iso);
+            ModAssert.True(ReferenceEquals(test.square, test.registry.Resolve(null)), "remaining implementation stays available");
+        }
+    }
+}