feat(xanthous): Fistfighting builds knuckle calluses

2000 ticks after the character damages their fists by hitting something,
the character now develops calluses on their fists (scaled by *how*
damaged they've become) that reduce the chance of them receiving
additional damage from hitting things - up to a max of 5, which
prevents *all* damage from fistfighting.

This is all tracked in a new "Knuckles" struct in a new "Body" struct on
the character datatype, which manages stepping itself forward as part of
the Brain impl on the character.

Change-Id: Ica269f16fb340fb25900d2c77fbad32f10c00be2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3222
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/xanthous/src/Xanthous/App.hs

@@ -362,10 +362,7 @@ attackAt pos =
         message msg msgParams
         entities . ix creatureID . positioned .= SomeEntity creature'
 
-    whenM (uses character $ isNothing . weapon)
-      $ whenM (chance (0.08 :: Float)) $ do
-        say_ ["combat", "fistSelfDamage"]
-        character %= Character.damage 1
+    whenM (uses character $ isNothing . weapon) handleFists
 
     stepGame -- TODO
   weapon chr = chr ^? inventory . wielded . wieldedItems . wieldableItem
@@ -377,6 +374,16 @@ attackAt pos =
       Nothing ->
         Messages.lookup ["combat", "hit", "fists"]
 
+  handleFists = do
+    damageChance <- use $ character . body . knuckles . to fistDamageChance
+    whenM (chance damageChance) $ do
+      damageAmount <- use $ character . body . knuckles . to fistfightingDamage
+      say_ [ "combat" , if damageAmount > 1
+                        then "fistExtraSelfDamage"
+                        else "fistSelfDamage" ]
+      character %= Character.damage damageAmount
+      character . body . knuckles %= damageKnuckles
+
 entityMenu_
   :: (Comonad w, Entity entity)
   => [w entity]