ink.branchline.createBookmark

Bookmark a bud. The caller must be signed in. If the caller already has a bookmark in the same story whose subject is an ancestor or descendant of the new subject, the existing record is rewritten in place (putRecord) so the bookmark list stays one-per-thread. Otherwise a new ink.branchline.bookmark record is created in the caller's PDS and mirrored into the local read cache.