Count what is unread from a marker the server cannot move

Unread was an integer on the buffer: bumped per arriving line, zeroed on
open, gone at exit. It could not survive what the server does. A JOIN
replays the backlog and the CHATHISTORY we ask for replays it again, so
every line came back through push-message! and ticked the counter a
second time — and none of it outlived the process anyway, so a restart
said every room was read whether or not it was.

So rooms and their read state go to ~/.config/frq/rooms.edn, and the
count is derived rather than kept. Each room records what it is, when it
last said anything, and how far into it the reader got: a msgid and the
time of it. `after-marker` takes what is past that — by id while that
line is still held, by time once it has aged out — and `recount` adds it
up. A replayed line is older than the marker and counts for nothing,
which is the whole point. Nothing writes a counter down, so there is no
number on disk that can be wrong.

The marker only goes forward. mark-read takes the max of what it had and
what arrived, because a backlog can land after the reader has already
read past it and taking the newest line's time flat would walk the
marker back and re-unread what was read.

Rooms are ours now, not the server's. It has told us we are in rooms we
are not and left out ones we are, so rooms.edn is what decides one
exists and its membership only sets a badge. channels.edn — names, no
markers — migrates in caught up as if read: the alternative announces a
hundred lines the reader has already seen.

Writes are off-thread and throttled to five seconds, forced when a room
is opened. The marker moves on every line that arrives while a room is
on screen and a busy channel would otherwise write per message. A late
write costs the handful of lines since the last one, shown unread again
next run — wrong in the direction that never claims to have read more
than it has.

Mentions ride along: recount marks a room whose unread lines carry the
reader's nick, and the row says `◆ @ 3` rather than `● 3`. That one flag
is cached on disk despite being derived — it is the only thing that
cannot be recomputed before the history it came from is back, so a room
that had your name in it says so on the first frame and is corrected
when the backlog lands.

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