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/TimingTests.cs

@@ -0,0 +1,65 @@
+using Engine.Core;
+
+namespace Engine.Tests;
+
+public class TimingTests
+{
+    [Fact]
+    public void Tick_Updates_DeltaTime()
+    {
+        var timing = new Timing();
+
+        timing.Tick();
+
+        Assert.True(timing.DeltaTime > 0);
+    }
+
+    [Fact]
+    public void Tick_Updates_TotalTime()
+    {
+        var timing = new Timing();
+
+        timing.Tick();
+        var time1 = timing.TotalTime;
+        timing.Tick();
+        var time2 = timing.TotalTime;
+
+        Assert.True(time2 > time1);
+    }
+
+    [Fact]
+    public void ConsumeFixedStep_Returns_True_When_Accumulator_Exceeds_Step()
+    {
+        var timing = new Timing { FixedTimeStep = 0.001 };
+
+        timing.Tick();
+        Thread.Sleep(5);
+        timing.Tick();
+
+        Assert.True(timing.ConsumeFixedStep());
+    }
+
+    [Fact]
+    public void ConsumeFixedStep_Returns_False_When_Below_Step()
+    {
+        var timing = new Timing { FixedTimeStep = 100.0 };
+
+        timing.Tick();
+
+        Assert.False(timing.ConsumeFixedStep());
+    }
+
+    [Fact]
+    public void ResetAccumulator_Clamps_To_Max()
+    {
+        var timing = new Timing { FixedTimeStep = 0.1 };
+
+        // Simulate a huge delta by ticking many times without consuming
+        for (var i = 0; i < 1000; i++)
+            timing.Tick();
+
+        timing.ResetAccumulator();
+
+        Assert.True(timing.FixedTimeAccumulator <= timing.FixedTimeStep * 5 + 0.001f);
+    }
+}