Fixes Defenders staying Fortified while being in critical state (#5641)

# About the pull request

Defenders who use Fortify and then get into critical are not pullable,
because Fortify is still technically active on them. Fortify only
automatically toggles itself off upon death. This PR changes it so the
Fortify ability gets turned off upon entering critical state, making
other xenos/yautja/synths able to pull them while they are downed.

Fixes #5616 

It also adds xeno crit comsig for other possible uses. I _think_ my
signal is only called once, when the xeno enters critical, but I may
have misread the code.

# Explain why it's good for the game

Fortified defenders are anchored until they are healed (and then they
manually turn it off) or until they die. This is probably unintended
behaviour, they are not conscious anymore to be fortified.

# Testing Photographs and Procedure

Spawned a defender, fortified, dealt it 500+ damage, then tried to drag
it. Video:

<details>
<summary>Screenshots & Videos</summary>


https://github.com/cmss13-devs/cmss13/assets/49321394/335f4d39-970d-4e90-a47b-801a152d8c37

</details>


# Changelog

:cl:
fix: Fixes Defenders staying Fortified in critical state - Fortify now
automatically toggles itself off upon the Defender getting critted.
/:cl:

code/__DEFINES/dcs/signals/atom/mob/living/signals_xeno.dm

@@ -71,3 +71,6 @@
 
 /// From /obj/effect/alien/resin/special/eggmorph/attack_alien: (mob/living/carbon/xenomorph/M)
 #define COMSIG_XENO_TAKE_HUGGER_FROM_MORPHER "xeno_take_hugger_from_morpher"
+
+/// From /mob/living/carbon/xenomorph/proc/handle_crit()
+#define COMSIG_XENO_ENTER_CRIT "xeno_entering_critical"

code/modules/mob/living/carbon/xenomorph/abilities/defender/defender_powers.dm

@@ -174,11 +174,13 @@
 	playsound(get_turf(xeno), 'sound/effects/stonedoor_openclose.ogg', 30, 1)
 
 	if(!xeno.fortify)
-		RegisterSignal(owner, COMSIG_MOB_DEATH, PROC_REF(death_check))
+		RegisterSignal(owner, COMSIG_XENO_ENTER_CRIT, PROC_REF(unconscious_check))
+		RegisterSignal(owner, COMSIG_MOB_DEATH, PROC_REF(unconscious_check))
 		fortify_switch(xeno, TRUE)
 		if(xeno.selected_ability != src)
 			button.icon_state = "template_active"
 	else
+		UnregisterSignal(owner, COMSIG_XENO_ENTER_CRIT)
 		UnregisterSignal(owner, COMSIG_MOB_DEATH)
 		fortify_switch(xeno, FALSE)
 		if(xeno.selected_ability != src)
@@ -249,9 +251,13 @@
 		else
 			damagedata["armor"] += frontal_armor
 
-/datum/action/xeno_action/activable/fortify/proc/death_check()
+/datum/action/xeno_action/activable/fortify/proc/unconscious_check()
 	SIGNAL_HANDLER
 
+	if(QDELETED(owner))
+		return
+
+	UnregisterSignal(owner, COMSIG_XENO_ENTER_CRIT)
 	UnregisterSignal(owner, COMSIG_MOB_DEATH)
 	fortify_switch(owner, FALSE)
 

code/modules/mob/living/carbon/xenomorph/life.dm

@@ -506,6 +506,7 @@ Make sure their actual health updates immediately.*/
 /mob/living/carbon/xenomorph/proc/handle_crit()
 	if(stat <= CONSCIOUS && !gibbing)
 		set_stat(UNCONSCIOUS)
+		SEND_SIGNAL(src, COMSIG_XENO_ENTER_CRIT)
 
 /mob/living/carbon/xenomorph/set_stat(new_stat)
 	. = ..()