Rename a block id AND every reference to it in the same document, in one id-boundary-safe write. Use this rather than geml_set or a text search-and-replace, which would also hit ids that merely share a prefix. A Markdown heading's anchor is its text, so it is renamed by changing the heading with geml_set (part: head), not here. An old id that does not exist, or a new one already taken, is refused. Returns {ok, file, diagnostics, revision}, with notes when the write did something to say out loud (a block it dropped, an address it changed); a refusal is ok: false with a hint, and the file is unchanged. 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.