Draw broken objects as fragments, plane tiles for fences and walls, tileset publishing; mod documentation

tests/RpgVisualGroupTests.cs

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+namespace Cthangover.Mods.Rpg.Tests
+{
+    using System.Text.Json;
+    using Cthangover.Core.Mods.Testing;
+    using Cthangover.Core.Utils;
+    /// <summary>Visual metadata survives the same serializer used by content and editor snapshots.</summary>
+    public class RpgVisualGroupTests
+    {
+        [ModTest("visual_groups/json_roundtrip")]
+        static void JsonRoundtrip(ModTestContext ctx)
+        {
+            var tileset = new RpgTileset { ID = "room" };
+            tileset.VisualGroups.Add(new RpgVisualGroup { Id = "wall", Width = 2, Height = 1, Pattern = new[] { 1000, 1001 },
+                AnchorX = 1, AnchorY = 0.75f, IsoPlanes = new() { new() { Crop = new[] { 0, 0, 2, 1 },
+                Points = new[] { new[] { 0f, 0, 1 }, new[] { 2f, 0, 1 }, new[] { 2f, 0, 0 }, new[] { 0f, 0, 0 } } } } });
+            var restored = JsonSerializer.Deserialize<RpgTileset>(JsonSerializer.Serialize(tileset), JsonOptionsProvider.ReadOptions);
+            var group = restored.VisualGroups[0];
+            ModAssert.Equal("wall", group.Id, "group ID");
+            ModAssert.Equal(2, group.Width, "width");
+            ModAssert.Equal(1, group.Height, "height");
+            ModAssert.Equal(1001, group.Pattern[1], "logical source IDs");
+            ModAssert.CloseTo(0.75, group.AnchorY, what: "pivot");
+            ModAssert.Equal(2, group.IsoPlanes[0].Crop[2], "plane crop");
+            ModAssert.CloseTo(1, group.IsoPlanes[0].Points[0][2], what: "elevation");
+            ModAssert.Equal(0, JsonSerializer.Deserialize<RpgTileset>("{}").VisualGroups.Count, "legacy default");
+        }
+    }
+}