Improve aim assist target stickiness

enemies/Enemy.gd

@@ -30,6 +30,10 @@ var max_hp: int
 
 func _ready() -> void:
 	add_to_group("enemies")
+	# Weapons discover anything the player may deliberately aim at through one
+	# compositional contract. Asteroids implement the same contract without the
+	# aim-assist code having to know either concrete type (#181).
+	add_to_group("player_aim_targets")
 	definition = ENEMY_DEFINITIONS.get_definition(enemy_id)
 	speed = definition.speed
 	acceleration = definition.acceleration