feat: 4x4 atlas with TileCoord-based UVs and grass faces

World/Blocks/GrassBlock.cs

@@ -9,8 +9,8 @@ public sealed class GrassBlock : IBlock
 
     public TileCoord GetTextureTile(BlockFace face) => face switch
     {
-        BlockFace.Top => new(0, 1),    // grass top
-        BlockFace.Bottom => new(0, 0), // dirt
-        _ => new(1, 1)                 // grass side
+        BlockFace.Top => new TileCoord(0, 1),    // grass top
+        BlockFace.Bottom => new TileCoord(0, 0), // dirt
+        _ => new TileCoord(1, 1)                 // grass side
     };
 }

World/Blocks/TileCoord.cs

@@ -0,0 +1,5 @@
+namespace VoxelEngine.World.Blocks;
+
+// Position of a tile within the texture atlas, in tile units.
+// Origin (0,0) is the top-left tile; X increases right, Y increases down.
+public readonly record struct TileCoord(int X, int Y);