The body was right-trimmed and then given a trailing newline back, which is
reasonable for a mail body and fatal for a published twtxt record: smolboid
refuses a post whose text carries a line break, because a record is one line
of a feed.
Trimming belongs here, before the seal, so the signature covers exactly the
text the author meant. A receiver cannot do it — stripping is not invertible,
so it could no longer rebuild the signed bytes to verify them.
