Follow the mihaela-agents mermaid rule: add a README roadmap using the TileDown
Apple-color status classes (done/review/epic/todo), legend block first, then a
libxml2-parity roadmap showing shipped vs open work. Add docs/rules/mermaid.md
and scripts/check-roadmap.sh (wired into check-all.sh) so the diagrams stay
mechanically checkable.
