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/MeshAndLightTests.cs

@@ -0,0 +1,40 @@
+using System.Numerics;
+using Engine.Core;
+using Engine.Core.Components;
+
+namespace Engine.Tests;
+
+public class MeshAndLightTests
+{
+    [Fact]
+    public void Mesh_Stores_Vertices_And_Indices()
+    {
+        var vertices = new[]
+        {
+            new Vertex(new Vector3(0, 0, 0), Vector3.One, Vector3.UnitY),
+            new Vertex(new Vector3(1, 0, 0), Vector3.One, Vector3.UnitY),
+            new Vertex(new Vector3(1, 1, 0), Vector3.One, Vector3.UnitY),
+        };
+        var indices = new uint[] { 0, 1, 2 };
+        var mesh = new Mesh(vertices, indices);
+
+        Assert.Equal(3, mesh.Vertices.Length);
+        Assert.Equal(3, mesh.Indices.Length);
+    }
+
+    [Fact]
+    public void Light_Direction_Is_Normalized()
+    {
+        var light = new Light(new Vector3(0, 2, 0), Vector3.One, 1.0f);
+
+        Assert.Equal(1f, light.Direction.Length(), 0.001f);
+    }
+
+    [Fact]
+    public void Light_With_Zero_Direction_Defaults_To_UnitY()
+    {
+        var light = new Light(Vector3.Zero, Vector3.One, 1.0f);
+
+        Assert.Equal(Vector3.UnitY, light.Direction);
+    }
+}