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/Streaming/ChunkJob.cs

@@ -3,12 +3,12 @@ using VoxelEngine.World.Chunk;
 
 namespace VoxelEngine.World.Streaming;
 
-// Job dispatched to an IJobExecutor. For a fresh load, ExistingChunk is null
-// and the worker generates + applies diff + meshes. For a remesh-only job,
-// ExistingChunk holds the chunk and the worker only meshes (no generation).
+// Job dispatched to an IJobExecutor for a fresh chunk load. The worker
+// generates the chunk, applies any modification diff, and meshes the result.
+// Remesh-only jobs are dispatched separately by DispatchRemesh and capture
+// their state via closures, so they do not use this record.
 internal sealed record ChunkJob(
     Vector3i Pos,
-    Chunk.Chunk? ExistingChunk,
     NeighborSnapshots Neighbors,
     IReadOnlyDictionary<int, ushort>? Diff);