Yetis available in snow.

src/time_system.nvgt

@@ -51,7 +51,7 @@ int zombie_swarm_duration_hours = 0;
 // Invasion mapping: "terrain=enemy" (defaults to bandits when no match)
 // Terrain keys: "mountain" for mountain ranges, or regular types like "grass", "snow", "forest", "deep_forest", "stone".
 string default_invasion_enemy_type = "bandit";
-string[] invasion_terrain_enemy_map = {"mountain=goblin"};
+string[] invasion_terrain_enemy_map = {"mountain=goblin", "snow=yeti"};
 
 void init_time() {
     current_hour = 8;