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/SettingsView.swift

@@ -11,10 +11,12 @@ import ARKit
 /// Zentraler Schlüssel für die LiDAR-Einstellung (in AppStorage/UserDefaults).
 enum SettingsKeys {
     static let lidarFeaturesEnabled = "lidarFeaturesEnabled"
+    static let arExposure = "arExposure"
 }
 
 struct SettingsView: View {
     @AppStorage(SettingsKeys.lidarFeaturesEnabled) private var lidarEnabled = false
+    @AppStorage(SettingsKeys.arExposure) private var arExposure = 1.0
     @Environment(\.dismiss) private var dismiss
 
     /// LiDAR ist verfügbar, wenn das Gerät Scene-Reconstruction (Mesh) unterstützt.
@@ -37,6 +39,23 @@ struct SettingsView: View {
                         Text("Dieses Gerät hat keinen LiDAR-Scanner. Die App nutzt automatisch das Verfahren ohne LiDAR.")
                     }
                 }
+
+                Section {
+                    HStack {
+                        Text("Belichtung")
+                        Spacer()
+                        Text(arExposure.formatted(.number.precision(.fractionLength(1))) + "×")
+                            .foregroundStyle(.secondary)
+                            .monospacedDigit()
+                    }
+                    Slider(value: $arExposure, in: 0.5...2.0, step: 0.1)
+                        .accessibilityLabel("AR-Belichtung")
+                        .accessibilityValue(arExposure.formatted(.number.precision(.fractionLength(1))) + " mal")
+                } header: {
+                    Text("Darstellung")
+                } footer: {
+                    Text("Passt die zusätzliche Beleuchtung der oberirdischen Tanks an. Änderungen greifen beim nächsten Start des AR-Modus.")
+                }
             }
             .navigationTitle("Einstellungen")
             .navigationBarTitleDisplayMode(.inline)