feat: add multi-chunk world with cross-chunk face culling

Add a World that stores chunks in a Dictionary keyed by chunk
coordinate, with floor-division world<->chunk coordinate mapping
(correct for negative coords). The mesher takes an optional neighbor
sampler so faces at chunk borders are culled against adjacent chunks
instead of always meshing the shell. Render one ChunkRenderer per
chunk, placed via the model matrix. Reorganize the World layer into
Blocks/ and Chunk/ sub-namespaces.

World/IWorld.cs

@@ -0,0 +1,11 @@
+using OpenTK.Mathematics;
+using VoxelEngine.World.Blocks;
+
+namespace VoxelEngine.World;
+
+public interface IWorld
+{
+    public void SetChunk(Vector3i chunkPos, Chunk.Chunk chunk);
+    public Chunk.Chunk? GetChunk(Vector3i chunkPos);
+    public IBlock GetBlock(int worldX, int worldY, int worldZ);
+}