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;
 

Assets/Scripts/ServerShared/Zone.cs

@@ -271,9 +271,6 @@ public class Zone
             var entity = EntitySerializer.Unpack(_itemManager, this, entityPack);
             var ship = entity as Ship;
             Admit(entity, piloted: ship is {IsPlayerShip: false});
-            // Generation placement, not admission: a ship packed at the origin is scattered across the zone.
-            if (ship is {IsPlayerShip: false} && lengthsq(ship.Position) < 1)
-                ship.Position = _itemManager.Random.NextFloat3(float3(-pack.Radius * .5f), float3(pack.Radius * .5f));
         }
 
         // TODO: Associate planets with stored entities for planetary colonies