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/Gameplay/ShipModVisual.cs

@@ -33,7 +33,7 @@ public static class ShipModVisual
     public static async Task<Instance> LoadAsync(string shipPath, Transform parent)
     {
         var package = ShipModCatalog.ReadPackage(shipPath);
-        var ship = package.Ship;
+        var ship = package.Visual;
 
         var root = new GameObject("mod-ship:" + ship.Id);
         root.SetActive(false);