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/modules/mob/living/carbon/human/human_update_icons.dm

@@ -64,7 +64,6 @@ There are several things that need to be remembered:
 	update_pockets()
 	update_worn_neck()
 	update_transform()
-	update_mutations_overlay()
 	update_damage_overlays()
 	// These are done via parent call update_body(), keeping them here for clarity
 	// update_hair()