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/MeshMath.cs

@@ -0,0 +1,25 @@
+using System.Numerics;
+
+namespace Engine.Graphics;
+
+/// <summary>
+/// Shared mesh math utilities used by loaders and procedural generators.
+/// </summary>
+public static class MeshMath
+{
+    /// <summary>
+    /// Compute a flat face normal from three vertex positions.
+    /// Falls back to Vector3.UnitY for degenerate (zero-area) triangles.
+    /// </summary>
+    public 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;
+    }
+}