Ship S1: one hull record, and a hull names its visual

A mod ship file now holds a HullData at mod-hull:<id> and the ShipAuthoring
visual package at mod-ship:<id>. HullData.Visual (key 32) is the typed ref
between them; ShipAuthoring.Hull (key 1) is retired.

- One joint validator over (hull, visual) is the only semantic check; Load
  owns the file structure (one hull, one visual, deterministic keys).
- Compose copies the mod store's records verbatim, so the derived catalog
  holds one hull per mod ship.
- ship_cc.py edits the hull record's slots directly; the body[HULL_SLOT]
  indirection is gone.
- The package fixture is generated from ShipAuthoringTests.Fixture() (glTF
  scene included) and exported when AETHERIA_SHIP_FIXTURE_DIR is set.

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

Assets/Scripts/ServerShared/ItemData.cs

@@ -530,6 +530,11 @@ public class HullData : EquippableItemData
     [Inspectable, JsonProperty("canTow"), Key(29)]
     public bool CanTow;
 
+    // The hull's visual: a ShipAuthoring record holding its model package, or unset for a Unity-prefab hull. A hull
+    // names exactly one of Prefab and Visual. Keys 30 and 31 are EquippableItemData's, so 32 is the next free key.
+    [JsonProperty("visual"), Key(32)]
+    public CultRecordRef<ShipAuthoring> Visual;
+
     [IgnoreMember]
     public Shape InteriorCells
     {