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/NavMeshWorldMessages.cs

@@ -0,0 +1,48 @@
+using UnityEngine;
+
+namespace InfiniteWorld.VoxelWorld.Contracts
+{
+    public readonly struct ChunkNavGeometryReadyMessage
+    {
+        public ChunkNavGeometryReadyMessage(Vector2Int coord, int version)
+        {
+            Coord = coord;
+            Version = version;
+        }
+
+        public Vector2Int Coord { get; }
+        public int Version { get; }
+    }
+
+    public readonly struct ChunkNavGeometryRemovedMessage
+    {
+        public ChunkNavGeometryRemovedMessage(Vector2Int coord, int version)
+        {
+            Coord = coord;
+            Version = version;
+        }
+
+        public Vector2Int Coord { get; }
+        public int Version { get; }
+    }
+
+    public readonly struct WorldInterestChangedMessage
+    {
+        public WorldInterestChangedMessage(int version)
+        {
+            Version = version;
+        }
+
+        public int Version { get; }
+    }
+
+    public readonly struct NavCoverageHintChangedMessage
+    {
+        public NavCoverageHintChangedMessage(int version)
+        {
+            Version = version;
+        }
+
+        public int Version { get; }
+    }
+}