Join a room already caught up on what was said before you

A room being joined for the first time has no marker, and a new buffer
started at zero — so the backlog the server replays on JOIN sat entirely
after the marker and was announced as unread. Being in a handful of busy
channels was enough to read as a hundred and one unread posts in one and
a hundred and ten in another, none of which anyone had missed: the
reader was not away for them, they were not there yet.

So a buffer nobody has seen before starts read rather than at the
beginning. To a minute ago, not to this instant: a live line is
timestamped by the server and this by our own clock, and where the two
disagree a message stamped a few seconds behind us would land under the
marker and never be counted at all. A minute is more skew than there
will be and far less than the age of any backlog. What it costs is that
a line said in the minute before you arrived counts as unread, which is
the direction that shows too much rather than too little.

Rooms that have been read are untouched — they come back from rooms.edn
with a marker of their own and never meet this default — and a DM that
arrives while nobody is looking still counts, because it is live and the
grace only reaches backwards.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>