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/TransformTests.cs

@@ -0,0 +1,52 @@
+using System.Numerics;
+using Engine.Core.Components;
+
+namespace Engine.Tests;
+
+public class TransformTests
+{
+    [Fact]
+    public void Identity_Transform_Produces_Identity_Matrix()
+    {
+        var t = new Transform(Vector3.Zero, Quaternion.Identity, Vector3.One);
+        var m = t.GetMatrix();
+
+        Assert.Equal(Matrix4x4.Identity, m);
+    }
+
+    [Fact]
+    public void Translation_Appears_In_Matrix()
+    {
+        var t = new Transform(new Vector3(1, 2, 3), Quaternion.Identity, Vector3.One);
+        var m = t.GetMatrix();
+
+        Assert.Equal(1f, m.M41);
+        Assert.Equal(2f, m.M42);
+        Assert.Equal(3f, m.M43);
+    }
+
+    [Fact]
+    public void Scale_Affects_Matrix_Diagonal()
+    {
+        var t = new Transform(Vector3.Zero, Quaternion.Identity, new Vector3(2, 3, 4));
+        var m = t.GetMatrix();
+
+        Assert.Equal(2f, m.M11);
+        Assert.Equal(3f, m.M22);
+        Assert.Equal(4f, m.M33);
+    }
+
+    [Fact]
+    public void Rotation_Around_Y_Rotates_X_Axis()
+    {
+        var angle = MathF.PI / 2f;
+        var rot = Quaternion.CreateFromAxisAngle(Vector3.UnitY, angle);
+        var t = new Transform(Vector3.Zero, rot, Vector3.One);
+        var m = t.GetMatrix();
+
+        var xAxis = new Vector3(m.M11, m.M21, m.M31);
+        Assert.Equal(0f, xAxis.X, 0.001f);
+        Assert.Equal(0f, xAxis.Y, 0.001f);
+        Assert.Equal(1f, xAxis.Z, 0.001f);
+    }
+}