feat(pathfinding): implement Phase 3 reactive pathing

- Add validation_cooldown field to Ambulatory component
- Implement validate_next_steps to check walkability of next 3 path nodes
- Integrate validation into movement system (every 10 frames)
- Trigger re-path when validation fails (path blocked or invalid)
- Entities now detect and recover from invalid paths automatically

src/entities/livestock/rabbit.rs

@@ -24,6 +24,7 @@ impl Rabbit {
                 current_path: None,
                 path_index: 0,
                 step_recovery: 0,
+                validation_cooldown: 0,
             },
             sprite: Sprite {
                 image: asset_server.load("rabbit.png"),