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/TankSelectionSection.swift

@@ -0,0 +1,30 @@
+//
+//  TankSelectionSection.swift
+//  GasTankAR
+//
+//  Created by Dennis Nemec on 14.09.26.
+//
+
+import SwiftUI
+
+/// Gemeinsame „Tankauswahl"-Section mit 3D-Vorschau und Editor. Wird sowohl beim
+/// Erstellen (`ProjectCreateView`) als auch beim Bearbeiten (`ProjectDetailView`)
+/// eines Projekts verwendet, damit Änderungen an einer Stelle gepflegt werden.
+struct TankSelectionSection: View {
+    @Binding var tank: Tank
+
+    var body: some View {
+        Section("Tankauswahl") {
+            TankPreview(tank: tank)
+
+            TankEditor(tank: $tank)
+        }
+    }
+}
+
+#Preview {
+    @Previewable @State var tank = Tank()
+    return Form {
+        TankSelectionSection(tank: $tank)
+    }
+}