[MIRROR] Makes Voidwalker check SSmapping.is_planetary when spawning (#29080)

* Makes Voidwalker check SSmapping.is_planetary when spawning (#85274)

## About The Pull Request

Per title. Dynamic naturally running will not spawn a voidwalker on
icebox now. ``find_space_spawn()`` checks only for carp landmarks, and
icebox has carp landmarks for the other midround invasion antags. As a
bonus, I also made SSpolling use the cosmic skull sprite to alert ghosts
with.

## Why It's Good For The Game

Just a lil' bug I found. Plus the skull sprite is cool as hell.

![image](https://github.com/user-attachments/assets/6459db22-f4a4-45b6-b770-8ec0d83bc379)

## Changelog

🆑
fix: Voidwalker should not run on planetary maps.
/🆑

* Makes Voidwalker check SSmapping.is_planetary when spawning

---------

Co-authored-by: The Sharkening <95130227+StrangeWeirdKitten@users.noreply.github.com>

code/controllers/subsystem/dynamic/dynamic_rulesets_midround.dm

@@ -972,13 +972,15 @@
 	cost = 5
 	minimum_players = 40
 	repeatable = TRUE
+	signup_item_path = /obj/item/cosmic_skull
 	ruleset_lazy_templates = list(LAZY_TEMPLATE_KEY_VOIDWALKER_VOID)
 	/// The space turf we find in acceptable(), cached for ease
 	var/space_turf
 
 /datum/dynamic_ruleset/midround/from_ghosts/voidwalker/acceptable(population = 0, threat_level = 0)
 	space_turf = find_space_spawn()
-	if(!space_turf)
+	// Space only antag and will die on planetary gravity.
+	if(SSmapping.is_planetary() || !space_turf)
 		return FALSE
 	return ..()