Replace ONE block and leave every other byte untouched — prefer this to rewriting a file. For content that does not exist yet use geml_add; to remove a block, geml_delete. The replacement is validated before it is written: if it would break the document, nothing is written and the diagnostics come back — fix the body rather than resending it. Removing content is not refused: if the replacement drops blocks, the write goes through and the result names each one, so check it after shortening a section; geml_revert puts one back. part replaces the whole block (default), its head line, a section's intro, or its body. In a Markdown file a heading's anchor is its text, as on GitHub: new heading text gives the heading a new address, and the document's links to the old one follow in the same write. GEML content written over a Markdown heading or prose is converted to Markdown, as in geml_add; a GEML block already in the file stays GEML. An id that matches no block, or several, 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.