refactor: unify item instance ids with the shared stable-id scheme

src/Features/Items/Godot/WorldItemController.cs

@@ -10,7 +10,7 @@ public partial class WorldItemController : RigidBody3D, IPickupTarget
 {
     public const string ItemGroup = "world_items";
 
-    [Export] public string InstanceId;
+    [Export] public string Key;
     [Export] public string DefinitionId = "item";
     [Export] public string DisplayName = "Item";
     [Export] public HandRequirement HandRequirement = HandRequirement.One;
@@ -41,13 +41,9 @@ public partial class WorldItemController : RigidBody3D, IPickupTarget
             throw new InvalidOperationException($"{Name}: held view scene is not assigned.");
         }
 
-        Guid id = string.IsNullOrWhiteSpace(InstanceId)
-            ? Guid.NewGuid()
-            : Guid.Parse(InstanceId);
-
         Definition = new ItemDefinition(new ItemDefinitionId(DefinitionId), DisplayName, HandRequirement);
         ItemInfo = new ItemInfo(
-            new ItemInstanceId(id),
+            ItemInstanceId.From(this.GetStableInstanceIdFrom(Key)),
             Definition.Id,
             Definition.DisplayName,
             Definition.HandRequirement