Rework maintenance loot spawners to fire at roundstart (#59913)

Maintenance loot spawners now fire at roundstart, rather than during the initialization of the Atoms subsystem.
Maintenance loot spawners do not qdel themselves during the spawning process, but persist, and merely hide themselves from living players.

As a cute side effect, this lets preround observers look at the colourful maintenance loot dice icons before the round starts. Observers can continue to see the loot dice post-round, but they are deliberately
very faint.

code/controllers/subsystem/mapping.dm

@@ -127,6 +127,7 @@ SUBSYSTEM_DEF(mapping)
 	setup_map_transitions()
 	generate_station_area_list()
 	initialize_reserved_level(transit.z_value)
+	SSticker.OnRoundstart(CALLBACK(src, .proc/spawn_maintenance_loot))
 	return ..()
 
 /datum/controller/subsystem/mapping/proc/wipe_reservations(wipe_safety_delay = 100)
@@ -590,3 +591,10 @@ GLOBAL_LIST_EMPTY(the_station_areas)
 		isolated_ruins_z = add_new_zlevel("Isolated Ruins/Reserved", list(ZTRAIT_RESERVED = TRUE, ZTRAIT_ISOLATED_RUINS = TRUE))
 		initialize_reserved_level(isolated_ruins_z.z_value)
 	return isolated_ruins_z.z_value
+
+/datum/controller/subsystem/mapping/proc/spawn_maintenance_loot()
+	for(var/obj/effect/spawner/lootdrop/maintenance/spawner as anything in GLOB.maintenance_loot_spawners)
+		CHECK_TICK
+
+		spawner.spawn_loot()
+		spawner.hide()