Validate a GEML document without changing it: returns every diagnostic with a stable code, a severity and a line, and an empty list means the document is valid. Use it to confirm a document is sound before reporting work as finished. Every write through this server runs the same check before it lands, so a refused write already carries this information. This server is STATELESS: the document travels in source and a write comes back as document for you to save; it holds no other documents, so a cross-document reference such as [[other.geml#id]] cannot be resolved here and is reported as unresolvable-document.