Fixes the healthdoll not showing robot limbs, or updating when pain goes away. (#22190)

Adds some signals to update the HUD when necessary.

The only one which will fire every tick is in remove_pain, which will
now only fire if there is pain to remove.

The extra update in robotize is necessary because robotize is called
after Initialize, so robot limbs were being set to the wrong colour.

Fixes: #22186

code/modules/mob/living/carbon/human/human_damage.dm

@@ -269,6 +269,8 @@
 		adjustToxLoss(getToxLoss()-amount)
 
 /mob/living/carbon/human/adjustHalLoss(var/amount)
+	if (!amount)
+		return
 	var/heal = (amount < 0)
 	amount = abs(amount)
 	var/list/pick_organs = organs.Copy()
@@ -279,7 +281,8 @@
 			continue
 
 		if(heal)
-			amount -= E.remove_pain(amount)
+			if(pain > 0)
+				amount -= E.remove_pain(amount)
 		else
 			amount -= E.add_pain(amount)
 	BITSET(hud_updateflag, HEALTH_HUD)