fix(rendering): preserve exact scene traversal order

src/AcDream.App/Rendering/Scene/RenderSceneContracts.cs

@@ -178,6 +178,35 @@ internal readonly record struct RenderDegradeState(byte Level, uint Revision);
 
 internal readonly record struct RenderSortKey(ulong Value);
 
+/// <summary>
+/// Read-only seam over the spatial owner's exact resident traversal. The
+/// render scene consumes this derived order only; it does not acquire spatial
+/// or gameplay ownership.
+/// </summary>
+internal interface IRenderTraversalOrderSource
+{
+    bool TryGetTraversalSortKey(
+        WorldEntity entity,
+        out RenderSortKey sortKey);
+}
+
+internal static class RenderTraversalSortKey
+{
+    public static RenderSortKey Compose(
+        uint landblockOrder,
+        int entityIndex)
+    {
+        if (landblockOrder == 0)
+            throw new ArgumentOutOfRangeException(nameof(landblockOrder));
+        if (entityIndex < 0)
+            throw new ArgumentOutOfRangeException(nameof(entityIndex));
+
+        return new RenderSortKey(
+            ((ulong)landblockOrder << 32)
+            | checked((uint)entityIndex));
+    }
+}
+
 /// <summary>
 /// Immutable source facts required to compare the derived presentation scene
 /// with the accepted current path. These are diagnostics/render inputs only;