Move rolling stock and physics into the domain

VehicleDatabase becomes VehicleCatalog, Physics becomes VehiclePhysics, and the
textures.txt and .fiz parsers move to Infrastructure. GameLibrary replaces the
GameData singleton.

StarterNG/Application/Abstractions/IPhysicsRepository.cs

@@ -0,0 +1,21 @@
+using StarterNG.Domain.Vehicles;
+
+namespace StarterNG.Application.Abstractions;
+
+/// <summary>
+/// Looks up the .fiz physics of a vehicle type by model name.
+/// </summary>
+public interface IPhysicsRepository
+{
+    /// <summary>Indexes the installation's .fiz files up front, off the UI thread.</summary>
+    void Preload();
+
+    /// <summary>How many vehicle types were indexed; zero means a broken installation.</summary>
+    int IndexedCount { get; }
+
+    /// <summary>
+    /// Physics for a model name (with or without extension), or null when the
+    /// installation has no .fiz for it.
+    /// </summary>
+    VehiclePhysics? For(string? dataFolder, string? model);
+}