determinism-ledger: delete the clock-seeded item stream and the item-stream scatter

ItemManager no longer seeds itself from DateTime.Now, and Zone's unpack no
longer scatters ships with _itemManager.Random. Deletes before adds: this
commit does not build (the scatter and every ItemManager seed return in the
next commit).

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

Assets/Scripts/ServerShared/ItemManager.cs

@@ -17,7 +17,7 @@ using float4 = CultMath.float4;
 
 public class ItemManager
 {
-    public Random Random = new Random((uint) (DateTime.Now.Ticks%uint.MaxValue));
+    public Random Random;
 
     private Action<string> _logger;