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/Features/VoxelWorld/Contracts/NavMeshWorldContracts.cs

@@ -17,6 +17,12 @@ namespace InfiniteWorld.VoxelWorld.Contracts
         void GetInterestPoints(List<WorldInterestPoint> results);
     }
 
+    public interface INavCoverageReader
+    {
+        bool IsPositionCovered(Vector3 worldPosition);
+        void GetCoverageWindows(List<NavCoverageWindowSnapshot> results);
+    }
+
     public readonly struct ChunkNavSourceSnapshot
     {
         public ChunkNavSourceSnapshot(Vector2Int coord, int version, ChunkNavBuildSourceDescriptor[] sources)
@@ -77,7 +83,31 @@ namespace InfiniteWorld.VoxelWorld.Contracts
     {
         PlayerActor = 0,
         ActiveNpc = 1,
-        Other = 2
+        SpawnAnchor = 2,
+        Other = 3
+    }
+
+    public readonly struct NavCoverageWindowSnapshot
+    {
+        public NavCoverageWindowSnapshot(int id, Bounds bounds, NavCoverageState state, int interestCount)
+        {
+            Id = id;
+            Bounds = bounds;
+            State = state;
+            InterestCount = interestCount;
+        }
+
+        public int Id { get; }
+        public Bounds Bounds { get; }
+        public NavCoverageState State { get; }
+        public int InterestCount { get; }
+    }
+
+    public enum NavCoverageState
+    {
+        Pending = 0,
+        Building = 1,
+        Ready = 2
     }
 
     public readonly struct ChunkNavGeometryReadyMessage