Cut C: delete ItemData.Manufacturer, derive census makers from products

The manufacturer does not belong to a design (operator ruling); it lives on
FactionProductData and branding is derived, never stored on the design.

- Delete ItemData.cs key 3 (Manufacturer), with a retired-key comment.
- AetherDb census now attributes a design's maker to the first product (in
  record-key order) that sells it, matching ItemManager.Brand's rule; a
  design no product sells shows "(none)".
- AetheriaStoresTests' catalog fixture moves its Manufacturer reference onto
  a FactionProductData selling the design, and FactionIsASingletonInstance
  reads it from there.
- mutation_tests.py gets a paired mutation for that fixture wiring.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Assets/Scripts/ServerShared/ItemData.cs

@@ -276,9 +276,9 @@ public abstract class ItemData
     
     [Inspectable, CultInspectorTextArea, JsonProperty("description"), Key(2)]
     public string Description;
-    
-    [Inspectable, JsonProperty("creator"), Key(3)]
-    public CultRecordRef<Faction> Manufacturer;
+
+    // MessagePack key 3 belonged to a removed creator/faction reference field; do not reuse it. That provenance
+    // now lives on FactionProductData, and branding is derived, never stored on the design.
 
     [Inspectable, JsonProperty("mass"), Key(4)]
     public float Mass;