## What changed
Add `codex-mermaid`, a standalone crate that renders supported subsets of flowchart, sequence, state, class, and ER diagrams as Unicode text. Expose `render` for plain text and `render_spans` for semantic node, edge, and text spans that callers can style.
Enforce source, diagram, canvas, and display-width limits. Return errors for unsupported input or exceeded limits without producing partial diagrams, leaving source fallback to callers. Include documentation and a stdin rendering example.
## Testing
Add snapshots for each diagram family, checks for relationship endpoints, Unicode labels, semantic spans, truncated input, and size limits, plus edge reconstruction for all 512 directed three-node graphs in all four layout directions.
GitOrigin-RevId: 5b4e65d9152abce5d833e4e6b12113a5a21f700e
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