feat: make Sand/Snow opaque and register Sand/Snow/Water blocks

VoxelEngine.Tests/BlockRegistryTests.cs

@@ -49,4 +49,26 @@ public class BlockRegistryTests
         Assert.Equal((ushort)2, registry.Dirt.Id);
         Assert.Equal((ushort)3, registry.Grass.Id);
     }
+
+    [Fact]
+    public void SandAndSnow_AreOpaque_WaterIsOpaque()
+    {
+        var registry = new BlockRegistry();
+        Assert.True(registry.Get(4).IsOpaque);  // Sand
+        Assert.True(registry.Get(5).IsOpaque);  // Snow
+        Assert.True(registry.Get(6).IsOpaque);  // Water (single-pass renderer => opaque)
+    }
+
+    [Fact]
+    public void Registry_ResolvesAllSevenBlocks()
+    {
+        var registry = new BlockRegistry();
+        Assert.Equal("Air", registry.Get(0).Name);
+        Assert.Equal("Stone", registry.Get(1).Name);
+        Assert.Equal("Dirt", registry.Get(2).Name);
+        Assert.Equal("Grass", registry.Get(3).Name);
+        Assert.Equal("Sand", registry.Get(4).Name);
+        Assert.Equal("Snow Block", registry.Get(5).Name);
+        Assert.Equal("Water", registry.Get(6).Name);
+    }
 }