Adds a overmap z failsafe (#2525)

nsv13/code/__DEFINES/overmap.dm

@@ -102,6 +102,9 @@ GLOBAL_LIST_INIT(overmap_impact_sounds, list('nsv13/sound/effects/ship/freespace
 #define INTERIOR_DELETING 3
 #define INTERIOR_DELETED 4
 
+//Overmap flags
+#define OVERMAP_FLAG_ZLEVEL_CARRIER (1<<0) //! This overmap is meant to carry a z with it, prompting restoration in certain cases.
+
 //Ship mass
 #define MASS_TINY 1 //1 Player - Fighters
 #define MASS_SMALL 2 //2-5 Players - FoB/Mining Ship

nsv13/code/modules/overmap/FTL/ftl_jump.dm

@@ -1,11 +1,17 @@
 /datum/star_system/proc/add_ship(obj/structure/overmap/OM, turf/target_turf)
 	if(!system_contents.Find(OM))
 		system_contents += OM	//Lets be safe while I cast some black magic.
+	var/did_restore_system = FALSE
 	if(!occupying_z && OM.z) //Does this system have a physical existence? if not, we'll set this now so that any inbound ships jump to the same Z-level that we're on.
 		if(!SSmapping.level_trait(OM.z, ZTRAIT_OVERMAP))
 			occupying_z = OM.get_reserved_z()
 		else
 			occupying_z = OM.z
+		did_restore_system = TRUE
+	else if(!occupying_z && ((OM.overmap_flags & OVERMAP_FLAG_ZLEVEL_CARRIER) || length(OM.mobs_in_ship))) //If someone is inside, or we always want it loaded, load it.
+		occupying_z = OM.get_reserved_z()
+		did_restore_system = TRUE
+	if(did_restore_system)
 		if(fleets.len)
 			for(var/datum/fleet/F in fleets)
 				if(!F.current_system)

nsv13/code/modules/overmap/ai-skynet.dm

@@ -1610,6 +1610,8 @@ Seek a ship thich we'll station ourselves around
 	var/ai_can_launch_fighters = FALSE //AI variable. Allows your ai ships to spawn fighter craft
 	var/list/ai_fighter_type = list()
 	var/ai_flags = AI_FLAG_DESTROYER
+	///Overmap bitflags
+	var/overmap_flags = NONE
 
 	var/list/holding_cargo = list() // list of objective datums. This station has cargo to deliver to the players as part of a courier objective
 	var/list/expecting_cargo = list() // list of objective datums. This station is expecting cargo delivered to them by the players as a part of a courier objective

nsv13/code/modules/overmap/overmap.dm

@@ -232,6 +232,7 @@ Proc to spool up a new Z-level for a player ship and assign it a treadmill.
 	var/turf/exit = get_turf(locate(round(world.maxx * 0.5, 1), round(world.maxy * 0.5, 1), world.maxz)) //Plop them bang in the center of the system.
 	var/obj/structure/overmap/OM = new _path(exit) //Ship'll pick up the info it needs, so just domp eet at the exit turf.
 	OM.reserved_z = world.maxz
+	OM.overmap_flags |= OVERMAP_FLAG_ZLEVEL_CARRIER
 	OM.current_system = SSstar_system.find_system(OM)
 	if(OM.role == MAIN_OVERMAP) //If we're the main overmap, we'll cheat a lil' and apply our status to all of the Zs under "station"
 		for(var/z in SSmapping.levels_by_trait(ZTRAIT_STATION))