Replace RPC with message-based protocol for web worker
- Add idl/message.ml with typed JSON message protocol for client-worker
communication (Init, Eval, Complete, TypeAt, Errors, CreateEnv, DestroyEnv)
- Rewrite lib/worker.ml to use message handlers instead of RPC server stubs
- Add multi-universe support to findlibish.ml for loading META files from
multiple findlib_index sources with cycle detection
- Update node test expected files for new output format
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>