Update an existing element by type and id, changing ONLY the fields you pass; omitted fields are kept.

This is the same partial update as PATCH /api/v2/{type}/{id}: each field you pass replaces that field's value whole. A text field you pass is set (pass an empty string "" to clear it). A multi-link field you pass REPLACES that field's whole array with the array you send (pass an empty array [] to clear it): a partial array drops every link it leaves out (only links your key cannot see are kept). To keep the rest, read the element first and send the whole array, or use edit_links, which adds or removes single links and leaves the rest of the array as it is. type is a slug from list_element_types; id is the element's UUID. Requires a WRITE API-Key. Returns the full updated element in the v2 wire shape. Errors if the element does not exist, or (naming the field) on an unknown field or a bad link target. On a world that takes an agent seat's writes as proposals, a seat's update is proposed, not applied: the answer is {proposed, proposal}, never the element, and a keeper (the owner or a co-builder) accepts or declines it. if_match (optional) is the element's change_seq as you read it: the change_seq field every v2 answer carries on the element, as PATCH /api/v2/{type}/{id} takes it in its If-Match header. The update applies only while the element is still at that number; if anyone wrote it since, nothing is written and the error's code is precondition_failed, with the element as it is now under current: re-read it, then send again or ask the user. Left out, the update applies whatever changed in between. A refusal from keel is the API's own error as JSON text, {"error": {message, type, code, param, doc_url}} (as the HTTP route answers it), with retry_after (seconds) beside it when the route would send Retry-After.