Add tentacled boss planet encounter

vfx/GasBombEffect.gd

@@ -1,6 +1,8 @@
 class_name GasBombEffect
 extends Node2D
 
+const COMBAT_TARGET = preload("res://core/CombatTarget.gd")
+
 # The gas bomb itself: the flight, the blast and the picture of both, with
 # nothing in it about who threw it.
 #
@@ -67,13 +69,11 @@ func _explode() -> void:
 	exploded = true
 	explosion_time = EXPLOSION_DURATION
 	hit_count = 0
-	# Asteroids too - a blast that clears everything around it but leaves the
-	# rocks untouched would need explaining, and they take damage the same way.
-	#
+	# Every weapon target opts into the same damage contract: ordinary enemies,
+	# asteroids, boss hatchlings and opened boss eyes all belong here.
 	# #176: player weapons never damage player buildings. A bomb may be thrown
 	# into a siege without punishing the structure it is meant to defend.
-	for target_node in get_tree().get_nodes_in_group("enemies") \
-			+ get_tree().get_nodes_in_group("asteroids"):
+	for target_node in get_tree().get_nodes_in_group(COMBAT_TARGET.GROUP):
 		if is_instance_valid(target_node) and target_node is Node2D \
 				and target_node.global_position.distance_to(target) <= AOE_RADIUS:
 			target_node.take_damage(damage)