refactor: store-owned diff snapshot and drop dead ChunkJob.ExistingChunk

TryGetDiff now returns an independent copy so callers can safely retain the
reference; removes the defensive copy from ChunkStreamer.DispatchLoad.
ChunkJob.ExistingChunk was never populated (DispatchRemesh uses closures
instead), so the field is removed along with its misleading record-level
comment.

World/Persistence/InMemoryChunkModificationStore.cs

@@ -10,7 +10,9 @@ public sealed class InMemoryChunkModificationStore : IChunkModificationStore
     {
         if (this._diffs.TryGetValue(chunkPos, out Dictionary<int, ushort>? d))
         {
-            diff = d;
+            // Returned snapshot: callers may safely retain and enumerate even
+            // after concurrent RecordEdit on the same chunk.
+            diff = new Dictionary<int, ushort>(d);
             return true;
         }
         diff = null;