Fixes GC for observers and new players, and tons of loose references to stuff that shouldnt be (#55563)

I've done this on a signal because atom_hud is applied very loosely in some contexes, and the objects themselves dont have an easy way to track back into what huds they're affected by, so this seemed the best. Properly cleans up next_time_allowed, which was missing.

Update: I discovered this is also an issue for a lot of simple mobs, such as slimes and bots and such. So fixing this is huge

code/datums/hud.dm

@@ -51,11 +51,14 @@ GLOBAL_LIST_INIT(huds, list(
 	GLOB.all_huds -= src
 	return ..()
 
-/datum/atom_hud/proc/remove_hud_from(mob/M)
+/datum/atom_hud/proc/remove_hud_from(mob/M, absolute = FALSE)
 	if(!M || !hudusers[M])
 		return
-	if (!--hudusers[M])
+	if (absolute || !--hudusers[M])
+		UnregisterSignal(M, COMSIG_PARENT_QDELETING)
 		hudusers -= M
+		if(next_time_allowed[M])
+			next_time_allowed -= M
 		if(queued_to_see[M])
 			queued_to_see -= M
 		else
@@ -81,6 +84,7 @@ GLOBAL_LIST_INIT(huds, list(
 		return
 	if(!hudusers[M])
 		hudusers[M] = 1
+		RegisterSignal(M, COMSIG_PARENT_QDELETING, .proc/unregister_mob)
 		if(next_time_allowed[M] > world.time)
 			if(!queued_to_see[M])
 				addtimer(CALLBACK(src, .proc/show_hud_images_after_cooldown, M), next_time_allowed[M] - world.time)
@@ -92,6 +96,10 @@ GLOBAL_LIST_INIT(huds, list(
 	else
 		hudusers[M]++
 
+/datum/atom_hud/proc/unregister_mob(datum/source, force)
+	SIGNAL_HANDLER
+	remove_hud_from(source, TRUE)
+
 /datum/atom_hud/proc/hide_single_atomhud_from(hud_user,hidden_atom)
 	if(hudusers[hud_user])
 		remove_from_single_hud(hud_user,hidden_atom)

code/modules/mob/dead/observer/observer.dm

@@ -160,6 +160,9 @@ GLOBAL_VAR_INIT(observer_default_invisibility, INVISIBILITY_OBSERVER)
 	addtimer(CALLBACK(src, /atom/proc/update_atom_colour), 10)
 
 /mob/dead/observer/Destroy()
+	if(data_huds_on)
+		remove_data_huds()
+
 	// Update our old body's medhud since we're abandoning it
 	if(mind?.current)
 		mind.current.med_hud_set_status()