reorganize navmesh contracts and services

Split VoxelWorld nav contracts into focused files and extract clustered coverage helpers so the navmesh service stays a coordinator instead of a catch-all runtime file.

Assets/Features/VoxelWorld/Contracts/NavMeshWorldEnums.cs

@@ -0,0 +1,18 @@
+namespace InfiniteWorld.VoxelWorld.Contracts
+{
+    public enum WorldInterestKind
+    {
+        PlayerActor = 0,
+        ActiveNpc = 1,
+        SpawnAnchor = 2,
+        TransientNavHint = 3,
+        Other = 4
+    }
+
+    public enum NavCoverageState
+    {
+        Pending = 0,
+        Building = 1,
+        Ready = 2
+    }
+}