feat: Implement NavMesh pathfinding and Deep Social visuals

- Phase 20-F: NavMesh Integration
  - Added 'com.unity.ai.navigation' package
  - Implemented Runtime NavMesh Baking in EnvironmentManager
  - Added NavMeshObstacle to environmental assets
  - Updated AgentVisual to use NavMeshAgent for movement
  - Implemented 'Instinctive Avoidance' via target offsetting

- Phase 21: Social Interaction & Expressions
  - Added procedural Dance and Wave animations in AgentAnimator
  - Implemented 'Dance Party' triggering logic in engine.py and AgentVisual
  - Added social relationship syncing (Backend -> Frontend)
  - Implemented proximity-based social greetings (Heart emote + Wave)
  - Updated Models.cs to support relationship data parsing

unity-client/Assets/Scripts/Models.cs

@@ -61,10 +61,24 @@ namespace TheIsland.Models
 
         // Shelter System (Phase 20-B)
         public bool is_sheltered;
+        
+        // Phase 21-B: Relationships Sync
+        public List<RelationshipData> relationships;
 
         public bool IsAlive => status == "Alive";
     }
 
+    /// <summary>
+    /// Relationship entry for Phase 21-B.
+    /// </summary>
+    [Serializable]
+    public class RelationshipData
+    {
+        public int target_id;
+        public string type;       // "friend", "close_friend", "rival"
+        public int affection;
+    }
+
     /// <summary>
     /// Agents update event data.
     /// </summary>