The persisted store was being overwritten with the windowed in-memory list, so
any channel or DM with more than _messageWindowSize (200) messages lost
everything older than the newest 200 on the first save after load. Observed
live: Public went 232 -> 201 in one session on a build that already had the
#333 fix, so this was not the race - it was windowing truncating the store.
This is the slow-erosion cause behind the whole 566 -> 231 -> 206 -> 201
history.
saveChannelMessages / saveMessages now MERGE into the persisted set instead of
overwriting: upsert by message identity so older persisted messages are kept,
new ones added, and the in-memory copy wins for edits/reactions/status. If the
existing history fails to decode, the save aborts loudly rather than merging
into an empty base and truncating (SAFELANE 6).
Deletion is now an explicit path - removeChannelMessage / removeMessage - since
the app deletes individual messages. Routing delete through the merging save
would resurrect them; the connector's deleteChannelMessage / deleteMessage now
call the explicit remove.
Windowing stays for display and memory; it no longer dictates what is stored.
Tests: 250-message history survives a 200-window save; new message appends;
edit is captured not duplicated; delete does not resurrect. 508 pass, analyze
and format clean.
Cost: a save now re-reads and re-encodes the channel/contact history. Cheap
with drift's per-key writes (#335); a future append-only schema removes even
that.
Strycher
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