Constrain lwt < 6.1.0 (6.1.1 incompatible with oxcaml)
lwt 6.1.1 fails to compile on oxcaml 5.2.0+ox due to type changes
in Unix.recv/send. The ox repo provides lwt.6.0.0+ox which works.
Adding an upper bound ensures the solver picks the right version.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>