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.Vulkan/VulkanRenderContext.cs

@@ -0,0 +1,35 @@
+using Engine.Core;
+using Engine.Graphics;
+
+namespace Engine.Graphics.Vulkan;
+
+/// <summary>
+/// Vulkan implementation of the render HAL context.
+/// Creates and owns an <see cref="Sdl3Window"/> for the Vulkan surface.
+/// </summary>
+public sealed class VulkanRenderContext : IRenderContext
+{
+    private readonly Sdl3Window _window;
+    private readonly VulkanContext _context;
+    private readonly Swapchain _swapchain;
+
+    public IWindow Window => _window;
+
+    public VulkanRenderContext(int width, int height, bool enableValidation)
+    {
+        _window = new Sdl3Window("Cortex Engine", width, height, vulkanSurface: true);
+        _context = new VulkanContext(_window, enableValidation);
+        _swapchain = new Swapchain(_context);
+    }
+
+    public IRenderer CreateRenderer() => new VulkanRenderer(_context, _swapchain);
+
+    public void Resize(int width, int height) => _swapchain.Recreate(width, height);
+
+    public void Dispose()
+    {
+        _swapchain.Dispose();
+        _context.Dispose();
+        _window.Dispose();
+    }
+}