feat: add block model with IBlock flyweight and registry

World/Blocks/GrassBlock.cs

@@ -0,0 +1,16 @@
+namespace VoxelEngine.World.Blocks;
+
+public sealed class GrassBlock : IBlock
+{
+    public ushort Id => 3;
+    public string Name => "Grass";
+    public bool IsOpaque => true;
+    public bool IsSolid => true;
+
+    public int GetTextureIndex(BlockFace face) => face switch
+    {
+        BlockFace.Top => 0,    // grass top
+        BlockFace.Bottom => 1, // dirt
+        _ => 3                 // grass side
+    };
+}