feat(runtime): seal dormant SetPosition evaluations

src/AcDream.Core/Physics/CellArray.cs

@@ -25,6 +25,14 @@ public sealed class CellArray : ICollection<uint>, IReadOnlyCollection<uint>
     private readonly List<uint> _order = new();
     private readonly HashSet<uint> _seen = new();
 
+    /// <summary>
+    /// Optional append-only union target used by one retained SetPosition
+    /// transaction. Clearing this CELLARRAY must not clear the target: retail
+    /// can rebuild the working array repeatedly while every probed cell still
+    /// contributes to the transaction's collision-world authority footprint.
+    /// </summary>
+    internal CellArray? UnionTarget { get; set; }
+
     public int Count => _order.Count;
     public bool IsReadOnly => false;
 
@@ -34,6 +42,11 @@ public sealed class CellArray : ICollection<uint>, IReadOnlyCollection<uint>
     /// <summary>Append <paramref name="id"/> iff not already present (retail add_cell dedup).</summary>
     public void Add(uint id)
     {
+        if (UnionTarget is { } target
+            && !ReferenceEquals(target, this))
+        {
+            target.Add(id);
+        }
         if (_seen.Add(id))
             _order.Add(id);
     }