Render Mermaid diagrams as Unicode box-drawing art

termaid renders a subset of Mermaid diagrams — flowcharts, state, class, entity-relationship, and sequence diagrams — into Unicode box-drawing art suitable for a terminal. Output is plain UTF-8 lines plus a TUI-agnostic class-tagged span model for callers that want to colorize; it depends on no terminal framework. Unsupported diagram types fall back to a framed raw box.

This is an OCaml port of the self-contained Rust mermaid.rs renderer from xai-org/grok-build, re-implemented and repackaged for opam.