Add tank catalog with USDZ models and tank selection

Introduce TankDefinition/TankCatalog loaded from Tanks.json, bundle the
Drachengas USDZ tank models, add tank selection and preview views, and
update the AR experience, editor and project views to use the catalog.
Includes tests for catalog loading and model selection.

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

GasTankAR/Views/TankAR/Measurement.swift

@@ -9,7 +9,23 @@ import Foundation
 import simd
 
 /// Eine benannte Strecke zwischen zwei im Raum gewählten Punkten (Welt-Koordinaten).
-struct Measurement: Identifiable {
+enum TankDimension: String, CaseIterable, Hashable, Identifiable {
+    case height
+    case length
+    case width
+
+    var id: Self { self }
+
+    var title: String {
+        switch self {
+        case .height: "Höhe"
+        case .length: "Länge"
+        case .width: "Breite"
+        }
+    }
+}
+
+struct Measurement: Identifiable, Codable {
     let id = UUID()
     var name: String
     let start: SIMD3<Float>