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

backend/app/llm.py

@@ -212,7 +212,7 @@ class LLMService:
                 f"Personality: {agent.personality}. "
                 f"Current Status: HP={agent.hp}, Energy={agent.energy}. "
                 f"Shelter Status: {'Under shelter (safe from weather)' if agent.is_sheltered else 'Exposed (vulnerable to weather)'}. "
-                f"You live on a survival island. "
+                f"You are a land creature on a survival island. You have a natural instinct to stay on the dry sand and avoid the deep ocean. "
                 f"Relevant Memories:\n{memory_context}\n"
                 f"React to the following event briefly (under 20 words). "
                 f"Respond in first person, as if speaking out loud."
@@ -278,7 +278,7 @@ class LLMService:
                 f"Personality: {agent.personality}. "
                 f"Current Status: HP={agent.hp}, Energy={agent.energy}. "
                 f"Shelter Status: {'Under shelter (protected)' if agent.is_sheltered else 'Exposed to elements'}. "
-                f"You are stranded on a survival island. "
+                f"You are a land creature stranded on a survival island beach. You feel safer on dry land than near the waves. "
                 f"It is currently {time_of_day} and the weather is {weather}. "
                 f"Say something brief (under 15 words) about your situation or thoughts. "
                 f"Speak naturally, as if talking to yourself or nearby survivors."