feat(VoxelGrid): add BTL-OFF rung to V-GRID setting

V-GRID now cycles OFF / ON / BTL-OFF.
BTL-OFF keeps the wireframe off during battles while leaving it
available in the overworld. ON preserves the original behaviour
(wireframe forced on in battles regardless of the row).

lib/BattleScene.lua

@@ -643,6 +643,10 @@ function BattleScene.render(state, arena, textures, token)
   -- the world (see VoxelGrid): the arena is drawn a unit per voxel like
   -- everything else, so the seams follow the one toggle and a player who
   -- turned them off does not get them back for the length of a fight.
+  local gridWas = VoxelGrid.override
+  if VoxelGrid.battleForced() then
+    VoxelGrid.override = true
+  end
   local out = nil
   local ok, err = pcall(function()
     -- its own canvas slot: this renders at the window's pixel size and the