Fix revived Pokemon not receiving experience share

When a Pokemon faints in battle, onFaint() clears it from the battle
participant set. The revive item restored HP but never re-added the
mon to self.participants, so at awardExp() time the revived mon passed
the HP check but failed the participant gate -- getting no exp.

Fix: re-add the revived mon to battle.participants in the revive item
effect, so it's counted as a participant and receives its share of
experience at battle end.

Fixes #648

src/inventory/ItemEffects.lua

@@ -324,6 +324,11 @@ function ItemEffects.use(data, save, itemId, target, battle, moveIndex, ow)
     require("src.core.Sound").play(data, "Heal_HP")
     -- a revive takes the same .healHP -> .doneHealing route, animating up
     -- from the fainted mon's 0 HP (#252)
+    -- re-add to participants so the revived mon gets its share of exp
+    -- at battle end (onFaint clears the flag; revive must restore it)
+    if battle and battle.participants then
+      battle.participants[target] = true
+    end
     return "consumed", { Strings("%s\nis revitalized!", monName(data, target)) },
            { healedFrom = 0 }
   end