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/VoxelGrid.lua

@@ -59,7 +59,7 @@ end
 
 -- where it persists and the rows that cycle it (see ModSetting)
 VoxelGrid.setting = ModSetting.new(VoxelGrid.KEY, VoxelGrid.LABEL,
-                                   { false, true }, { "OFF", "ON" })
+                                   { false, true, "btloff" }, { "OFF", "ON", "BTL-OFF" })
 
 -- The row is the whole answer, everywhere: free-roam and the battle arena
 -- alike. The battle used to force the seams on regardless -- a fight is a
@@ -68,7 +68,14 @@ VoxelGrid.setting = ModSetting.new(VoxelGrid.KEY, VoxelGrid.LABEL,
 -- whole mod, and a mode that came back for every fight read as the row not
 -- working rather than as a deliberate framing.
 function VoxelGrid.enabled()
-  return VoxelGrid.setting:get() and true or false
+  if VoxelGrid.override ~= nil then return VoxelGrid.override end
+  local v = VoxelGrid.setting:get()
+  if v == "btloff" then return false end
+  return v and true or false
+end
+
+function VoxelGrid.battleForced()
+  return VoxelGrid.setting:get() ~= "btloff"
 end
 
 function VoxelGrid.set(enabled, game)
@@ -80,7 +87,7 @@ function VoxelGrid.toggle(game)
 end
 
 function VoxelGrid.sync(value)
-  VoxelGrid.setting:sync(value and true or false)
+  VoxelGrid.setting:sync(value)
 end
 
 function VoxelGrid.row()