fix(streaming): player entity no longer disappears on landblock unload

ROOT CAUSE: MarkPersistent(chosen.Id) stored the SERVER GUID (e.g.
0x5000000A) but RemoveLandblock checked entity.Id which is the LOCAL
sequential counter (e.g. 42). Different number spaces → never matched
→ persistent rescue never triggered → player entity lost on unload.

Fix:
- Added WorldEntity.ServerGuid field (0 for dat-hydrated scenery)
- Live entity spawn sets ServerGuid = spawn.Guid
- RemoveLandblock checks entity.ServerGuid against _persistentGuids
- MarkPersistent still stores the server GUID (correct)

This bug has been reported across multiple sessions as "character
disappears when walking far." The neverCullLandblockId fix only
prevented frustum culling but didn't prevent the entity from being
removed from the render list entirely.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -767,6 +767,7 @@ public sealed class GameWindow : IDisposable
         var entity = new AcDream.Core.World.WorldEntity
         {
             Id = _liveEntityIdCounter++,
+            ServerGuid = spawn.Guid,
             SourceGfxObjOrSetupId = spawn.SetupTableId.Value,
             Position = worldPos,
             Rotation = rot,