ALL stamped rows folded into this block, hidden anchors included.
Timestamp of the session's last stamped row (≈ when the transcript ended).
The last visible turn's text (untruncated — the card ellipsizes), or null.
Who spoke the last visible turn (AI → Assistant), or null when no visible turn exists.
MJ: AI Agent Sessions.ID the block's rows are stamped with (original casing of the first row).
Timestamp of the session's first stamped row (≈ when the session's transcript began).
VISIBLE caption turns: non-hidden User/AI rows with text (mirrors review-mode's turn mapping).
One realtime session's collapsed block: everything the timeline card renders that can be computed from the conversation's already-loaded detail rows (no extra queries).