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

World/Blocks/BlockRegistry.cs

@@ -8,11 +8,15 @@ public sealed class BlockRegistry
     public StoneBlock Stone { get; } = new();
     public DirtBlock Dirt { get; } = new();
     public GrassBlock Grass { get; } = new();
+    public SandBlock Sand { get; } = new();
+    public SnowBlock Snow { get; } = new();
+    public WaterBlock Water { get; } = new();
 
     public BlockRegistry()
     {
         // Index by id. Order does not matter; slot = block.Id.
-        IBlock[] blocks = new IBlock[] { this.Air, this.Stone, this.Dirt, this.Grass };
+        IBlock[] blocks = new IBlock[]
+            { this.Air, this.Stone, this.Dirt, this.Grass, this.Sand, this.Snow, this.Water };
         this._byId = new IBlock[blocks.Length];
         foreach (IBlock block in blocks)
         {