FireControl.Extract stamps every mined stack with the Extracted lot of its zone, belt and ore

Extracted gains Body at key 2. ItemManager.ExtractedLot finds or mints the lot; its
(zone, belt, ore) index is derived from the ledger at construction and never saved.
SavedGame roots skip null items and Lot 0.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Assets/Scripts/ServerShared/SavedGame.cs

@@ -225,7 +225,7 @@ public static class RunSave
         var roots = zones
             .SelectMany(zone => zone.Contents?.Entities ?? new List<EntityPack>())
             .SelectMany(EntitySerializer.Items)
-            .Where(item => item.Lot != 0)
+            .Where(item => item != null && item.Lot != 0)
             .Select(item => item.Lot);
         cache.Commit(batch =>
         {