Layers entangled mobs above spacevines (#92432)

## About The Pull Request
Changes a mobs layer to `SPACEVINE_MOB_LAYER` when they get tangled in
spacevines. Reverts it when they unbuckle.

## Why It's Good For The Game
Makes mobs vulnerable to man-eating plants when they foolishly get
entangled.
Its pretty annoying to bite buckled mobs at their moment of
vulnerability when playing as venus human trap. There aren't many pixels
for you to click, and you instantly destroy the spacevine structure if
you miss.

## Changelog
🆑
qol: Mobs entangled within spacevines are layered above the spacevines
for as long as they stay buckled
/🆑

code/modules/events/space_vines/vine_mutations.dm

@@ -12,6 +12,20 @@
 	holder.mutations |= src
 	holder.add_atom_colour(hue, FIXED_COLOUR_PRIORITY)
 
+/datum/spacevine_mutation/proc/on_buckle(obj/structure/spacevine/holder, mob/living/buckled)
+	SHOULD_CALL_PARENT(TRUE)
+	buckled.layer = SPACEVINE_MOB_LAYER
+	RegisterSignal(buckled, COMSIG_MOB_UNBUCKLED, PROC_REF(on_unbuckle))
+
+/datum/spacevine_mutation/proc/on_unbuckle(datum/source)
+	SHOULD_CALL_PARENT(TRUE)
+	SIGNAL_HANDLER
+	if(!isliving(source))
+		return
+	var/mob/living/buckled = source
+	buckled.layer = initial(buckled.layer)
+	UnregisterSignal(buckled, COMSIG_MOB_UNBUCKLED)
+
 /datum/spacevine_mutation/proc/process_mutation(obj/structure/spacevine/holder)
 	return
 
@@ -39,9 +53,6 @@
 /datum/spacevine_mutation/proc/on_spread(obj/structure/spacevine/holder, turf/target)
 	return
 
-/datum/spacevine_mutation/proc/on_buckle(obj/structure/spacevine/holder, mob/living/buckled)
-	return
-
 /datum/spacevine_mutation/proc/on_explosion(severity, target, obj/structure/spacevine/holder)
 	return FALSE
 
@@ -168,6 +179,7 @@
 
 /// What happens if an aggr spreading vine buckles a mob.
 /datum/spacevine_mutation/aggressive_spread/on_buckle(obj/structure/spacevine/holder, mob/living/buckled)
+	. = ..()
 	aggrospread_act(holder, buckled)
 
 /// Hurts mobs. To be used when a vine with aggressive spread mutation spreads into the mob's tile or buckles them.