The new command note.split {story, id, at} splits an editorial note at
byte `at` of its text. The note keeps the text before that point, and
the rest moves into a new note by the same author whose anchor directly
follows the first one. A split point at the start or end of the text,
or inside a character, is refused. The Notes panel shows each note's
text in a read-only field, and its Split Note button splits the note at
the insertion point in that field.