Explore mermaid diagrams / language / URL pattern → img/data encoding/decoding

- https://mermaid.ink/ URL pattern + img encoding from diagram
- also supported in github `readme.md` files https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams
- https://mermaid.live/ live editor

Start by reviewing the mermaid.ink URL pattern and its image encoding, then compare the GitHub README diagram support and mermaid.live editor links. The issue does not name repository files or tests; done would require defining the intended URL, language, and encoding/decoding behavior before implementation.

Written by the indexing model from the issue text.