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

tests/Engine.Tests/CameraTests.cs

@@ -0,0 +1,65 @@
+using System.Numerics;
+using Engine.Core.Components;
+
+namespace Engine.Tests;
+
+public class CameraTests
+{
+    [Fact]
+    public void View_Matrix_Transforms_Position_To_Origin()
+    {
+        var cam = new Camera(
+            new Vector3(0, 0, 10),
+            new Vector3(0, 0, 0),
+            Vector3.UnitY,
+            MathF.PI / 4f,
+            16f / 9f,
+            0.1f,
+            100f);
+
+        var view = cam.GetViewMatrix();
+        var originInCameraSpace = Vector3.Transform(new Vector3(0, 0, 0), view);
+
+        Assert.Equal(0f, originInCameraSpace.X, 0.001f);
+        Assert.Equal(0f, originInCameraSpace.Y, 0.001f);
+        Assert.Equal(-10f, originInCameraSpace.Z, 0.001f);
+    }
+
+    [Fact]
+    public void Projection_Matrix_Has_Correct_Aspect_Ratio()
+    {
+        var cam = new Camera(
+            Vector3.Zero,
+            Vector3.UnitZ,
+            Vector3.UnitY,
+            MathF.PI / 4f,
+            16f / 9f,
+            0.1f,
+            100f);
+
+        var proj = cam.GetProjectionMatrix();
+
+        Assert.True(proj.M11 > 0);
+        Assert.True(proj.M22 > 0);
+        Assert.Equal(0f, proj.M41, 0.001f);
+    }
+
+    [Fact]
+    public void Default_Material_Has_Expected_Values()
+    {
+        var mat = Material.Default;
+
+        Assert.Equal(0.5f, mat.Roughness);
+        Assert.Equal(0.0f, mat.Metallic);
+        Assert.False(mat.HasTexture);
+    }
+
+    [Fact]
+    public void Material_With_Texture_Path_Has_Texture_Flag()
+    {
+        var mat = new Material(texturePath: "Content/test.png");
+
+        Assert.True(mat.HasTexture);
+        Assert.Equal("Content/test.png", mat.TexturePath);
+    }
+}