Every USER message starts a conversation (the same boundary the turn
engine's delta scoping keys on). The latest conversation renders in full;
older ones collapse to a one-line summary row — head text, reply count,
last-activity time — that expands/collapses on click. db's 'threads per
conversation' ask, delivered as timeline folding: no log-model change, no
new persistence, cross-machine sync untouched.
groupChatConversations() splits the log (leading member run from a trimmed
log forms a headless block); rendering extracts renderEntry() unchanged.
Tests: conversation splitting (headless block, heads, startIndex) + fold
source contracts. 256/256 plugin tests pass.
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