Refuse re-entrant, fractional-level and no-party wild battles

overworld() resolves the world from under the stack, so a call from a
battle hook stacked a second battle over the live one -- on a loss its
afterBattle blacked out and warped with the outer battle still up.

newWild marks the species SEEN before it reports an empty party, so a
refused call still wrote the Pokedex; test the party before building it.

tonumber accepts 5.5, which Pokemon.new writes straight into the stat
calc and the exp curve.

src/world/WorldAPI.lua

@@ -7,6 +7,7 @@
 
 local Logger = require("src.core.Logger")
 local MapLoader = require("src.world.MapLoader")
+local Party = require("src.pokemon.Party")
 local Runtime = require("src.mods.Runtime")
 
 local WorldAPI = {}
@@ -174,13 +175,34 @@ function WorldAPI:startWildBattle(species, level)
   if not self.game.data.pokemon[species] then
     return nil, "unknown species: " .. tostring(species)
   end
+  -- Pokemon.new writes the level through verbatim -- into level, the stat
+  -- calc and the exp curve -- so a fraction has to be refused here rather
+  -- than round somewhere downstream.  The % test also catches NaN, which
+  -- passes both range comparisons.
   level = tonumber(level)
-  if not level or level < 1 or level > 100 then
-    return nil, "level must be 1..100"
+  if not level or level % 1 ~= 0 or level < 1 or level > 100 then
+    return nil, "level must be a whole number 1..100"
+  end
+  -- overworld() resolves the world from UNDER whatever sits on top of it,
+  -- so from a battle hook this would otherwise stack a second battle over
+  -- the live one -- and on a loss its afterBattle blacks out and warps
+  -- with the outer battle still on the stack.
+  local BattleTransition = require("src.render.BattleTransition")
+  for _, state in ipairs(self.game.stack and self.game.stack.states or {}) do
+    if state.awardExp or getmetatable(state) == BattleTransition then
+      return nil, "a battle is already running"
+    end
+  end
+  if ow.transitioning then return nil, "the world is mid-warp" end
+  -- BattleState.newWild marks the species SEEN before it reports an empty
+  -- party, so the party check comes first: a refused call must not leave a
+  -- Pokedex entry behind.
+  local save = self.game.save
+  if not (save and Party.firstHealthy(save.party or {})) then
+    return nil, "no healthy party"
   end
   local battle = require("src.battle.BattleState")
     .newWild(self.game, species, level)
-  if battle.dead then return nil, "no healthy party" end
   battle.onFinish = function(result) ow:afterBattle(result, battle) end
   ow:pushBattle(battle)
   return true