THANSHEER

Safely change the Mermaid Flow parse↔serialize round-trip core. Use when editing parser.ts, serializer.ts, model.ts, layout.ts, or adding a node shape / edge kind / direction / Mermaid syntax — anywhere a change could drop or corrupt a user's diagram. Encodes the non-negotiable invariants and the test loop so AI-generated changes to this fragile area stay safe.