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.

Core/Configuration.cs

@@ -10,6 +10,11 @@ public sealed class Configuration
     public int MaxMeshUploadsPerFrame { get; set; } = 4;
     public int WorkerThreadCount { get; set; } = Math.Max(1, Environment.ProcessorCount / 2);
 
+    // Multiplier applied to (dy)^2 in chunk-load priority. >1 prioritizes
+    // chunks at the player's Y over equally-distant chunks above/below, so the
+    // visible surface fills in before far-underground / overhead chunks.
+    public int VerticalPriorityWeight { get; set; } = 4;
+
     public float MoveSpeed { get; set; } = 8f;
     public float MouseSensitivity { get; set; } = 0.15f;
     public float Fov { get; set; } = 60f;