Standing overlay cleanup (#96593)

## About The Pull Request

Every standing overlay layer that wasn't actually a standing overlay
layer is now a "sub-standing layer", denoted with a decimal

Total number of standing overlay layers reduced from 39 to 23

Small refactors to mutation overlay/species handling and cult halo. 

## Why It's Good For The Game

Makes it easier to work with mob overlays. 

You can see at a glance which layers are applied with standing overlays
and which layers are manually handled, and more importantly you can see
how the layers sort themselves out by just looking at them. Very
convenient.

## Changelog

🆑 Melbert
code: Reorganized human layering, report any oddities with sprites, such
as mislayered equipment.
refactor: Refactored cult halos, report any oddities like them not
appearing when they should.
refactor: Refactored mutation visuals, they're now affected by height!
Report any oddities like them disappearing.
refactor: Refactored mutation species handling, report any oddities like
lizards with hulk.
/🆑

---------

Co-authored-by: Ghom <42542238+Ghommie@users.noreply.github.com>

code/datums/elements/cult_halo.dm

@@ -1,53 +0,0 @@
-/**
- * # Cult halo element
- *
- * Applies and removes the cult halo
- */
-/datum/element/cult_halo
-
-/datum/element/cult_halo/Attach(datum/target, initial_delay = 20 SECONDS)
-	. = ..()
-	if (!isliving(target))
-		return ELEMENT_INCOMPATIBLE
-
-	// Register signals for mob transformation to prevent premature halo removal
-	RegisterSignals(target, list(COMSIG_CHANGELING_TRANSFORM, COMSIG_MONKEY_HUMANIZE, COMSIG_HUMAN_MONKEYIZE), PROC_REF(set_halo))
-	addtimer(CALLBACK(src, PROC_REF(set_halo), target), initial_delay)
-
-/**
- * Halo setter proc
- *
- * Adds the cult halo overlays, and adds the halo trait to the mob.
- */
-/datum/element/cult_halo/proc/set_halo(mob/living/target)
-	SIGNAL_HANDLER
-
-	if(!IS_CULTIST(target))
-		target.RemoveElement(/datum/element/cult_halo)
-		return
-
-	ADD_TRAIT(target, TRAIT_CULT_HALO, CULT_TRAIT)
-	var/mutable_appearance/new_halo_overlay = mutable_appearance('icons/mob/effects/halo.dmi', "halo[rand(1, 6)]", -HALO_LAYER)
-	if (ishuman(target))
-		var/mob/living/carbon/human/human_parent = target
-		new /obj/effect/temp_visual/cult/sparks(get_turf(human_parent), human_parent.dir)
-		human_parent.overlays_standing[HALO_LAYER] = new_halo_overlay
-		human_parent.apply_overlay(HALO_LAYER)
-	else
-		target.add_overlay(new_halo_overlay)
-
-/**
- * Detach proc
- *
- * Removes the halo overlays, and trait from the mob
- */
-/datum/element/cult_halo/Detach(mob/living/target, ...)
-	REMOVE_TRAIT(target, TRAIT_CULT_HALO, CULT_TRAIT)
-	if (ishuman(target))
-		var/mob/living/carbon/human/human_parent = target
-		human_parent.remove_overlay(HALO_LAYER)
-		human_parent.update_body()
-	else
-		target.cut_overlay(HALO_LAYER)
-	UnregisterSignal(target, list(COMSIG_CHANGELING_TRANSFORM, COMSIG_HUMAN_MONKEYIZE, COMSIG_MONKEY_HUMANIZE))
-	return ..()