fix(unread): read timeline mentions when catching up at the bottom - #1679

## Problem

Bug report (mobile 0.28.0 (46)): opening a mention from the Inbox in an
archived Solo chat leaves the Inbox row unread.

The video shows the cause. Each unread row is a long agent answer. On a
phone, the answer is taller than the screen, or it lands partly cut off
after the deep-link jump. Mobile automatic reading writes a `msg:` mark
only for a row that is fully visible for the 300 ms dwell. A mention row
is high priority, so `activity:<channel>` does not read it. The mention
can stay unread until the reader marks it read by hand.

The archived state is not the cause. A widget test shows that an
archived Solo chat writes read marks for short messages. Archived chats
make the problem easy to see, because there is no composer and the
reader only reads.

## Fix

A message counts as read only when it actually showed, for the 300 ms
dwell. A row counts when any of these holds:

- **It fits:** the whole row is inside the reading area (unchanged).
- **It fills the screen:** the row is taller than the reading area and
covers all of it. This applies to the channel timeline and the thread
view.
- **A link opened it:** the message a deep link or Inbox tap opened
counts once any part of it shows, because the jump places it at 35% and
a long one lands cut off.
- **It is the newest message at the live bottom:** its end is showing,
in a channel, a Solo chat (replies inline) or a thread. This covers a
long final answer that only ever shows its end.

Reaching the bottom does **not** read mentions or broadcasts above the
newest message that never came on screen. Desktop does that sweep
(block/buzz-app#1679). Mobile deliberately does not. Ordinary
`activity:` / `thread-activity:` catch-up is unchanged, and messages the
reader marked unread stay unread.

## Tests

- `reading_marks_test.dart`: the bottom does not read mentions above it;
the newest message is read at the live bottom (and respects manual
unread); a flat Solo timeline reads only its newest inline reply; a tall
thread tail reply is read at its end; DM catch-up does not mark
mentions; `isReadableRow` cases, including linked rows.
- `channel_detail_page_test.dart`: an archived Solo chat and a Solo
inline reply each read a tall newest mention at the bottom; a tall row
away from the bottom is read once it fills the screen; tall linked
messages (channel and thread) are read when the jump leaves them cut
off; in a thread, the tall last reply is read at the end and an earlier
tall reply is read once it fills the screen. These fail on `main`.
- `just mobile-check` and the full mobile suite pass.

## Human test steps

1. Get a long agent reply that mentions you in a Solo chat, then archive
the chat.
2. Open the row from the Inbox and hold still for about one second.
3. Go back. The row should now show as read, with no dot.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Signed-off-by: Blake McAnally <[email protected]>
Signed-off-by: morgmart <[email protected]>
Co-authored-by: BlakeBot <85bbd70443139fd5c5762e34c2e214240939eb243c37f98380f69fed3b3bb4a6@buzz.block.builderlab.xyz>
Co-authored-by: Claude Opus 5.5 <[email protected]>
Co-authored-by: morgmart <[email protected]>
Co-authored-by: Morgarita <6e967af659416160ab4f2fd56d74c8c6f273791ee7bfc9468a1be37c387947be@buzz.block.builderlab.xyz>