React to a message in a group and only its author sees it. A quiet note
without turning it into an event for everybody else.
The reaction is sent to the author and to nobody else, which is the whole
guarantee: asking the other members' apps not to display it would be a
request, and a request is not privacy. The envelope carries a flag so the
author's app can label what it got, not to ask anyone to hide anything.
Private reactions are kept apart from the public tally on both devices
that hold them, and drawn with a label, so one is never mistaken for
something the group watched arrive.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
