feat: add face-culled chunk mesher behind IChunkMesher

VoxelEngine.Tests/ChunkMesherTests.cs

@@ -0,0 +1,55 @@
+using VoxelEngine.World;
+using Xunit;
+
+namespace VoxelEngine.Tests;
+
+public class ChunkMesherTests
+{
+    private const int FloatsPerFace = 36; // 6 verts * 6 floats
+
+    private static int FaceCount(float[] mesh) => mesh.Length / FloatsPerFace;
+
+    [Fact]
+    public void EmptyChunk_ProducesNoFaces()
+    {
+        var mesh = new ChunkMesher().Build(new Chunk(), new BlockRegistry());
+        Assert.Empty(mesh);
+    }
+
+    [Fact]
+    public void SingleBlockSurroundedByAir_Produces6Faces()
+    {
+        var chunk = new Chunk();
+        chunk[8, 8, 8] = 1; // stone
+
+        var mesh = new ChunkMesher().Build(chunk, new BlockRegistry());
+
+        Assert.Equal(6, FaceCount(mesh));
+    }
+
+    [Fact]
+    public void FullySolidChunk_ProducesOnlyOuterShell()
+    {
+        var chunk = new Chunk();
+        for (int x = 0; x < Chunk.Size; x++)
+        for (int y = 0; y < Chunk.Size; y++)
+        for (int z = 0; z < Chunk.Size; z++)
+            chunk[x, y, z] = 1; // stone
+
+        var mesh = new ChunkMesher().Build(chunk, new BlockRegistry());
+
+        // 6 sides * 16 * 16 visible faces
+        Assert.Equal(6 * Chunk.Size * Chunk.Size, FaceCount(mesh));
+    }
+
+    [Fact]
+    public void EveryVertexHasSixFloats()
+    {
+        var chunk = new Chunk();
+        chunk[0, 0, 0] = 1;
+
+        var mesh = new ChunkMesher().Build(chunk, new BlockRegistry());
+
+        Assert.Equal(0, mesh.Length % 6);
+    }
+}