Xenomorph embryos now grow based on consistent timers instead of 100% RNG (3% chance every 2s) (#55755)

• Adds documentation to alien_embryo.dm
• Refactors how alien embryos increase their stage var, aka their cute ickle baby development stage. Now it increases stage every minute, which means 5 minutes to grow as opposed to taking 15 seconds to the end of time to grow.
• Growth time is now defined by a variable, so admins can var-edit it in game and future coders can easily change the variable to make it take more or less time to grow. It also means it's a simple one-line edit if any of TG's plentiful downstreams want to change this for their server.

code/modules/mob/living/carbon/alien/special/alien_embryo.dm

@@ -5,12 +5,20 @@
 	icon = 'icons/mob/alien.dmi'
 	icon_state = "larva0_dead"
 	food_reagents = list(/datum/reagent/consumable/nutriment = 5, /datum/reagent/toxin/acid = 10)
+	///What stage of growth the embryo is at. Developed embryos give the host symptoms suggesting that an embryo is inside them.
 	var/stage = 0
+	/// Are we bursting out of the poor sucker who's the xeno mom?
 	var/bursting = FALSE
+	/// How long does it take to advance one stage? Growth time * 5 = how long till we make a Larva!
+	var/growth_time = 60 SECONDS
+
+/obj/item/organ/body_egg/alien_embryo/Initialize()
+	. = ..()
+	advance_embryo_stage()
 
 /obj/item/organ/body_egg/alien_embryo/on_find(mob/living/finder)
 	..()
-	if(stage < 4)
+	if(stage < 5)
 		to_chat(finder, "<span class='notice'>It's small and weak, barely the size of a foetus.</span>")
 	else
 		to_chat(finder, "<span class='notice'>It's grown quite large, and writhes slightly as you look at it.</span>")
@@ -20,7 +28,7 @@
 /obj/item/organ/body_egg/alien_embryo/on_life()
 	. = ..()
 	switch(stage)
-		if(2, 3)
+		if(3, 4)
 			if(prob(2))
 				owner.emote("sneeze")
 			if(prob(2))
@@ -29,7 +37,7 @@
 				to_chat(owner, "<span class='danger'>Your throat feels sore.</span>")
 			if(prob(2))
 				to_chat(owner, "<span class='danger'>Mucous runs down the back of your throat.</span>")
-		if(4)
+		if(5)
 			if(prob(2))
 				owner.emote("sneeze")
 			if(prob(2))
@@ -46,12 +54,17 @@
 			to_chat(owner, "<span class='danger'>You feel something tearing its way out of your stomach...</span>")
 			owner.adjustToxLoss(10)
 
-/obj/item/organ/body_egg/alien_embryo/egg_process()
-	if(stage < 5 && prob(3))
-		stage++
+/// Controls Xenomorph Embryo growth. If embryo is fully grown (or overgrown), stop the proc. If not, increase the stage by one and if it's not fully grown (stage 6), add a timer to do this proc again after however long the growth time variable is.
+/obj/item/organ/body_egg/alien_embryo/proc/advance_embryo_stage()
+	if(stage >= 6)
+		return
+	if(++stage < 6)
 		INVOKE_ASYNC(src, .proc/RefreshInfectionImage)
+		addtimer(CALLBACK(src, .proc/advance_embryo_stage), growth_time)
 
-	if(stage == 5 && prob(50))
+
+/obj/item/organ/body_egg/alien_embryo/egg_process()
+	if(stage == 6 && prob(50))
 		for(var/datum/surgery/S in owner.surgeries)
 			if(S.location == BODY_ZONE_CHEST && istype(S.get_surgery_step(), /datum/surgery_step/manipulate_organs))
 				AttemptGrow(0)
@@ -59,7 +72,6 @@
 		AttemptGrow()
 
 
-
 /obj/item/organ/body_egg/alien_embryo/proc/AttemptGrow(gib_on_success=TRUE)
 	if(!owner || bursting)
 		return
@@ -73,7 +85,8 @@
 
 	if(!candidates.len || !owner)
 		bursting = FALSE
-		stage = 4
+		stage = 5	// If no ghosts sign up for the Larva, let's regress our growth by one minute, we will try again!
+		addtimer(CALLBACK(src, .proc/advance_embryo_stage), growth_time)
 		return
 
 	var/mob/dead/observer/ghost = pick(candidates)