Materials And Stack Harddel (#22313)

To clarify I have recently cancelled my subscription to google's coding
agents, so this is my first hard del PR in several weeks that was done
completely by hand. This PR fixes a hard del on Materials (and their
parent Stacks), which was being caused by up to 4 different vars
potentially containing references that were not being cleared during
their Destroy() procs.

I have also tested this by hand, by spawning and deleting stacks, then
waiting to see if the hard del triggers or not.

---------

Signed-off-by: VMSolidus <evilexecutive@gmail.com>

code/game/objects/items/stacks/stack.dm

@@ -62,6 +62,10 @@
 	if (src && usr && usr.machine == src)
 		usr << browse(null, "window=stack")
 		usr.unset_machine()
+
+	recipes = null
+	build_type = null
+	synths = null
 	return ..()
 
 /obj/item/stack/update_icon()