Editar artifact

Applies patch operations to the head and writes a new version. Operations: replace (old, new), insert_before/insert_after (anchor, text), replace_range (from, to, new), prepend/append (text), and selector operations on one element of the content region: set_text (selector, text), replace_element (selector, html), set_attribute (selector, name, value), remove_element (selector). Anchors and selectors must match exactly once. Send base_version to get 409 ARTIFACT_STALE instead of editing a newer head.