The codec (Set.Marshal/Unmarshal) and its posix_lock.proto were built to
let the lock set ride in an inode's entry metadata, but the authority is
in-memory and ownership handoff/restart is handled by mounts re-asserting
their held locks over the RPC — neither serializes the set. Nothing calls
the serde outside its own tests, so drop it (codec, proto, generated pb,
Makefile). The in-memory Set/Manager are unchanged.