Fix some signal registration issues with arm implants (#95510)

## About The Pull Request

- `COMSIG_CARBON_POST_ATTACH_LIMB` and `COMSIG_KB_MOB_DROPITEM_DOWN`
were not being unregistered on limb removal, leading to duplicate
registration runtimes if the same limb was amputated and then reattached
- `/obj/item/organ/cyberimp/arm/on_limb_attached` early returns if the
attached limb isn't in the implant's body zone i.e. it's unrelated, the
child override `/obj/item/organ/cyberimp/arm/toolkit/on_limb_attached`
calls the parent but doesn't early return if the parent did, leading to
erroneous signal registration on unrelated limbs

code/modules/surgery/organs/internal/cyberimp/augments_arms.dm

@@ -22,12 +22,16 @@
 
 /obj/item/organ/cyberimp/arm/on_mob_remove(mob/living/carbon/arm_owner)
 	. = ..()
+	UnregisterSignal(arm_owner, COMSIG_CARBON_POST_ATTACH_LIMB)
 	on_limb_detached(hand)
 
 /obj/item/organ/cyberimp/arm/proc/on_limb_attached(mob/living/carbon/source, obj/item/bodypart/limb)
 	SIGNAL_HANDLER
 	if(!limb || QDELETED(limb) || limb.body_zone != zone)
 		return
+	handle_attachment(limb)
+
+/obj/item/organ/cyberimp/arm/proc/handle_attachment(obj/item/bodypart/limb)
 	if(hand)
 		on_limb_detached(hand)
 	RegisterSignal(limb, COMSIG_BODYPART_REMOVED, PROC_REF(on_limb_detached))
@@ -88,9 +92,10 @@
 
 /obj/item/organ/cyberimp/arm/toolkit/on_mob_remove(mob/living/carbon/arm_owner)
 	. = ..()
+	UnregisterSignal(arm_owner, COMSIG_KB_MOB_DROPITEM_DOWN)
 	Retract()
 
-/obj/item/organ/cyberimp/arm/toolkit/on_limb_attached(mob/living/carbon/source, obj/item/bodypart/limb)
+/obj/item/organ/cyberimp/arm/toolkit/handle_attachment(obj/item/bodypart/limb)
 	. = ..()
 	RegisterSignal(limb, COMSIG_ITEM_ATTACK_SELF, PROC_REF(on_item_attack_self))