feat: add block model with IBlock flyweight and registry

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() { }
+}