Final-diff pass: trimGroupChatLog drops entries from the FRONT once a room
crosses the history cap, so slicing the post-turn log at the pre-turn
LENGTH could overshoot after a mid-turn trim, read an empty tail, and
silently commit a stale turn — re-opening #93127's double delivery in
long-history rooms exactly. Anchor on the last pre-turn entry's id; if the
anchor itself was trimmed, every surviving entry is newer, so scanning the
whole log stays exact.
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