fix(physics): activate collision generations atomically

src/AcDream.App/Streaming/LandblockPresentationPipeline.cs

@@ -215,6 +215,18 @@ public sealed class LandblockPresentationPipeline
     public IReadOnlyList<LandblockStreamResult> GetPendingPublicationResults() =>
         _publications.Keys.ToArray();
 
+    /// <summary>
+    /// Cancels retained publication receipts during a generation reset. A
+    /// collision receipt owns only its private staging world until activation,
+    /// so cancellation cannot withdraw or partially replace the active world.
+    /// </summary>
+    internal void CancelPendingPublications()
+    {
+        foreach (PublicationTransaction transaction in _publications.Values)
+            transaction.PhysicsPublication?.Dispose();
+        _publications.Clear();
+    }
+
     public void ResumePublication(LandblockStreamResult result)
     {
         ArgumentNullException.ThrowIfNull(result);