refactor nav coverage into clustered windows

Replace region-based runtime pathing with interest-cluster coverage windows so active nav areas stay contiguous and spawn anchors participate in initial coverage.

Assets/Scripts/VoxelWorld/VoxelWorldSpawnAnchor.cs

@@ -0,0 +1,12 @@
+using UnityEngine;
+
+namespace VoxelWorldScene
+{
+    [DisallowMultipleComponent]
+    public sealed class VoxelWorldSpawnAnchor : MonoBehaviour
+    {
+        [SerializeField, Min(0.01f)] private float priority = 2f;
+
+        public float Priority => priority;
+    }
+}