feat: add fly-camera with view/projection matrices

Rendering/Camera.cs

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+using OpenTK.Mathematics;
+
+namespace VoxelEngine.Rendering;
+
+public sealed class Camera
+{
+    private static readonly Vector3 WorldUp = Vector3.UnitY;
+
+    public Vector3 Position { get; private set; }
+    public float Yaw { get; private set; } = -90f;   // degrees; -90 looks down -Z
+    public float Pitch { get; private set; } = 0f;    // degrees
+    public float Fov { get; set; } = 60f;             // degrees
+    public float AspectRatio { get; set; }
+
+    public Vector3 Front { get; private set; } = -Vector3.UnitZ;
+    public Vector3 Right { get; private set; } = Vector3.UnitX;
+    public Vector3 Up { get; private set; } = Vector3.UnitY;
+
+    public Camera(Vector3 position, float aspectRatio)
+    {
+        Position = position;
+        AspectRatio = aspectRatio;
+        UpdateVectors();
+    }
+
+    public void Move(Vector3 delta) => Position += delta;
+
+    public void AddRotation(float yawDelta, float pitchDelta)
+    {
+        Yaw += yawDelta;
+        Pitch = MathHelper.Clamp(Pitch + pitchDelta, -89f, 89f);
+        UpdateVectors();
+    }
+
+    public Matrix4 GetViewMatrix() => Matrix4.LookAt(Position, Position + Front, Up);
+
+    public Matrix4 GetProjectionMatrix() =>
+        Matrix4.CreatePerspectiveFieldOfView(
+            MathHelper.DegreesToRadians(Fov), AspectRatio, 0.1f, 100f);
+
+    private void UpdateVectors()
+    {
+        float yaw = MathHelper.DegreesToRadians(Yaw);
+        float pitch = MathHelper.DegreesToRadians(Pitch);
+
+        Front = Vector3.Normalize(new Vector3(
+            MathF.Cos(yaw) * MathF.Cos(pitch),
+            MathF.Sin(pitch),
+            MathF.Sin(yaw) * MathF.Cos(pitch)));
+
+        Right = Vector3.Normalize(Vector3.Cross(Front, WorldUp));
+        Up = Vector3.Normalize(Vector3.Cross(Right, Front));
+    }
+}

VoxelEngine.Tests/CameraTests.cs

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+using OpenTK.Mathematics;
+using VoxelEngine.Rendering;
+using Xunit;
+
+namespace VoxelEngine.Tests;
+
+public class CameraTests
+{
+    [Fact]
+    public void DefaultFront_PointsTowardNegativeZ()
+    {
+        var camera = new Camera(Vector3.Zero, 16f / 9f);
+
+        Assert.Equal(0f, camera.Front.X, 3);
+        Assert.Equal(0f, camera.Front.Y, 3);
+        Assert.Equal(-1f, camera.Front.Z, 3);
+    }
+
+    [Fact]
+    public void Move_TranslatesPosition()
+    {
+        var camera = new Camera(Vector3.Zero, 1f);
+        camera.Move(new Vector3(1f, 2f, 3f));
+        Assert.Equal(new Vector3(1f, 2f, 3f), camera.Position);
+    }
+
+    [Fact]
+    public void AddRotation_ClampsPitchTo89Degrees()
+    {
+        var camera = new Camera(Vector3.Zero, 1f);
+        camera.AddRotation(0f, 200f); // try to over-rotate up
+        Assert.True(camera.Pitch <= 89.0001f);
+        camera.AddRotation(0f, -400f);
+        Assert.True(camera.Pitch >= -89.0001f);
+    }
+
+    [Fact]
+    public void ProjectionMatrix_IsNotIdentity()
+    {
+        var camera = new Camera(Vector3.Zero, 1.5f);
+        Assert.NotEqual(Matrix4.Identity, camera.GetProjectionMatrix());
+    }
+}