Cobalt is a fork of LuaJ 2.0 (Lua 5.1) with many features of LuaJ 3.0 backported.
It allows multiple Lua instances to be run at once, with no shared metatables.
LuaJ 2.0 had too many bugs, mostly minor but annoying. Cobalt is an attempt to slim down LuaJ (only the JSE will be supported) and fix most of the bugs.
I am considering having a separate Lua 5.3 branch but that is not an immediate priority right now.