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/Loaders/GltfLoader.cs

@@ -81,14 +81,5 @@ public static class GltfLoader
     }
 
     private static Vector3 ComputeFaceNormal(Vector3 a, Vector3 b, Vector3 c)
-    {
-        var ab = b - a;
-        var ac = c - a;
-        var normal = Vector3.Cross(ab, ac);
-        if (normal.LengthSquared() > 0.00001f)
-            normal = Vector3.Normalize(normal);
-        else
-            normal = Vector3.UnitY;
-        return normal;
-    }
+        => MeshMath.ComputeFaceNormal(a, b, c);
 }

src/Engine.Graphics/Loaders/ObjLoader.cs

@@ -84,14 +84,5 @@ public static class ObjLoader
     }
 
     private static Vector3 ComputeFaceNormal(Vector3 a, Vector3 b, Vector3 c)
-    {
-        var ab = b - a;
-        var ac = c - a;
-        var normal = Vector3.Cross(ab, ac);
-        if (normal.LengthSquared() > 0.00001f)
-            normal = Vector3.Normalize(normal);
-        else
-            normal = Vector3.UnitY;
-        return normal;
-    }
+        => MeshMath.ComputeFaceNormal(a, b, c);
 }