feat: modular HAL, Raylib backend, PBR shading, textures, 60 unit tests

- Replace hardcoded SDL3 windowing with IWindow/IInputState/Key abstractions
- Each render backend owns its window (Raylib GLFW, SDL3 for Vulkan)
- Raylib backend: DrawModelEx, custom GLSL shader with Fresnel, ACES
  tonemapping, gamma correction, hemisphere ambient
- Fix backface culling, mesh memory (NativeMemory.Alloc), texture loading
- Camera controllers use backend-agnostic Key enum (inverted yaw/strafe)
- Demo scene: 8 cubes, 7 spheres, torus knot OBJ with checker texture
- Extract ProceduralMesh + MeshMath from Program.cs to Engine.Graphics
- Vulkan backend deferred (compiles, untested, IWindow-compatible)
- 60 unit tests: ObjLoader, camera controllers, AiCommandProcessor,
  RenderBackendFactory, Timing, ProceduralMesh, MeshMath, Transform
- AGENTS.md for opencode integration

src/Engine.Graphics/RenderBackendFactory.cs

@@ -0,0 +1,36 @@
+using Engine.Core;
+
+namespace Engine.Graphics;
+
+/// <summary>
+/// Factory for creating concrete graphics backends by name.
+/// Backends register themselves so the app only depends on the HAL interfaces.
+/// Each backend creates and owns its own window.
+/// </summary>
+public static class RenderBackendFactory
+{
+    private static readonly Dictionary<string, Func<int, int, bool, IRenderContext>> _registry
+        = new(StringComparer.OrdinalIgnoreCase);
+
+    /// <summary>
+    /// Register a backend implementation under the given name.
+    /// The factory receives (width, height, enableValidation) and must create
+    /// its own window and render context.
+    /// </summary>
+    public static void Register(string name, Func<int, int, bool, IRenderContext> factory)
+    {
+        _registry[name] = factory;
+    }
+
+    /// <summary>
+    /// Create a backend instance for the given name.
+    /// The backend assembly must have registered itself before this is called.
+    /// </summary>
+    public static IRenderContext Create(string name, int width, int height, bool enableValidation)
+    {
+        if (!_registry.TryGetValue(name, out var factory))
+            throw new NotSupportedException($"No graphics backend named '{name}' is registered.");
+
+        return factory(width, height, enableValidation);
+    }
+}