feat: add block model with IBlock flyweight and registry

World/BlockFace.cs

@@ -0,0 +1,11 @@
+namespace VoxelEngine.World;
+
+public enum BlockFace
+{
+    Top,    // +Y
+    Bottom, // -Y
+    North,  // -Z
+    South,  // +Z
+    East,   // +X
+    West    // -X
+}

World/BlockRegistry.cs

@@ -0,0 +1,26 @@
+using VoxelEngine.World.Blocks;
+
+namespace VoxelEngine.World;
+
+public sealed class BlockRegistry
+{
+    private readonly IBlock[] _byId;
+
+    public AirBlock Air { get; } = new();
+    public StoneBlock Stone { get; } = new();
+    public DirtBlock Dirt { get; } = new();
+    public GrassBlock Grass { get; } = new();
+
+    public BlockRegistry()
+    {
+        // Index by id. Order does not matter; slot = block.Id.
+        var blocks = new IBlock[] { Air, Stone, Dirt, Grass };
+        _byId = new IBlock[blocks.Length];
+        foreach (IBlock block in blocks)
+        {
+            _byId[block.Id] = block;
+        }
+    }
+
+    public IBlock Get(ushort id) => _byId[id];
+}

World/Blocks/AirBlock.cs

@@ -0,0 +1,10 @@
+namespace VoxelEngine.World.Blocks;
+
+public sealed class AirBlock : IBlock
+{
+    public ushort Id => 0;
+    public string Name => "Air";
+    public bool IsOpaque => false;
+    public bool IsSolid => false;
+    public int GetTextureIndex(BlockFace face) => 0;
+}

World/Blocks/DirtBlock.cs

@@ -0,0 +1,10 @@
+namespace VoxelEngine.World.Blocks;
+
+public sealed class DirtBlock : IBlock
+{
+    public ushort Id => 2;
+    public string Name => "Dirt";
+    public bool IsOpaque => true;
+    public bool IsSolid => true;
+    public int GetTextureIndex(BlockFace face) => 1;
+}

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
+    };
+}

World/Blocks/StoneBlock.cs

@@ -0,0 +1,10 @@
+namespace VoxelEngine.World.Blocks;
+
+public sealed class StoneBlock : IBlock
+{
+    public ushort Id => 1;
+    public string Name => "Stone";
+    public bool IsOpaque => true;
+    public bool IsSolid => true;
+    public int GetTextureIndex(BlockFace face) => 2;
+}

World/IBlock.cs

@@ -0,0 +1,21 @@
+namespace VoxelEngine.World;
+
+public interface IBlock
+{
+    ushort Id { get; }
+    string Name { get; }
+
+    // Drives face culling: a face is emitted only if its neighbor is not opaque.
+    bool IsOpaque { get; }
+
+    // Reserved for future collision (not used in the baseline).
+    bool IsSolid { get; }
+
+    // Atlas tile index for the given face.
+    int GetTextureIndex(BlockFace face);
+
+    // Interaction hooks — capability present now, system built post-baseline.
+    void OnInteract() { }
+    void OnPlaced() { }
+    void OnBroken() { }
+}