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/TankCatalogEnvironment.swift

@@ -0,0 +1,28 @@
+//
+//  TankCatalogEnvironment.swift
+//  GasTankAR
+//
+//  Created by Dennis Nemec on 14.09.26.
+//
+
+import SwiftUI
+
+private struct TankCatalogKey: EnvironmentKey {
+    static let defaultValue: any TankCatalogProviding = TankCatalog.shared
+}
+
+extension EnvironmentValues {
+    /// Der in der View-Hierarchie verfügbare Tank-Katalog. Standard: der aus dem
+    /// Bundle geladene `TankCatalog.shared`; in Tests/Previews überschreibbar.
+    var tankCatalog: any TankCatalogProviding {
+        get { self[TankCatalogKey.self] }
+        set { self[TankCatalogKey.self] = newValue }
+    }
+}
+
+extension View {
+    /// Injiziert einen Tank-Katalog in die View-Hierarchie.
+    func tankCatalog(_ catalog: any TankCatalogProviding) -> some View {
+        environment(\.tankCatalog, catalog)
+    }
+}