merlint fixes: .ocamlformat, Fmt.failwith, unused field names

Eio HTTP server with static file serving and route handlers

- Add missing .ocamlformat to 4 packages (E500)
- stix: use Fmt.failwith instead of failwith (Fmt.str ...) (E700)
- mbr: rename _unused1/_unused2 fields — underscore prefix implies
unused but they're stored in the record (E726)