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/IRenderer.cs

@@ -0,0 +1,31 @@
+using Engine.Core;
+using Flecs.NET.Core;
+
+namespace Engine.Graphics;
+
+/// <summary>
+/// Renders the ECS world and exposes screenshot capture.
+/// Implemented by concrete graphics backends.
+/// </summary>
+public interface IRenderer : IDisposable
+{
+    /// <summary>
+    /// Render one frame of the ECS world and present it.
+    /// </summary>
+    void RenderWorld(World world);
+
+    /// <summary>
+    /// Request a screenshot of the next rendered frame to be saved to disk.
+    /// </summary>
+    void RequestScreenshot(string outputPath);
+
+    /// <summary>
+    /// True if a screenshot has been requested but not yet captured.
+    /// </summary>
+    bool IsScreenshotRequested { get; }
+
+    /// <summary>
+    /// Provider that can asynchronously capture the current frame to PNG bytes.
+    /// </summary>
+    IScreenshotProvider ScreenshotProvider { get; }
+}