feat: weight vertical distance in chunk-load priority

Adds Configuration.VerticalPriorityWeight (default 4). The priority
formula in ChunkStreamer.RecomputeTargetSet now uses
dx^2 + dz^2 + dy^2 * VerticalPriorityWeight, so horizontal chunks at
the player's Y are dispatched before equally-distant chunks above or
below. Fixes janky loading where deep-underground chunks were
competing with surface chunks for early dispatch slots.

World/Streaming/ChunkStreamer.cs

@@ -122,9 +122,12 @@ public sealed class ChunkStreamer : IDisposable
             if (this._failed.Contains(pos)) continue;
             if (this._tracked.Add(pos))
             {
-                int priority = (pos.X - p.X) * (pos.X - p.X)
-                             + (pos.Y - p.Y) * (pos.Y - p.Y)
-                             + (pos.Z - p.Z) * (pos.Z - p.Z);
+                int dx = pos.X - p.X;
+                int dy = pos.Y - p.Y;
+                int dz = pos.Z - p.Z;
+                int priority = dx * dx
+                             + dz * dz
+                             + dy * dy * this._cfg.VerticalPriorityWeight;
                 this._pendingLoad.Enqueue(pos, priority);
             }
         }