Marker scale was deferred in a resolved review thread but only the
compaction-progress half was written down. Deferred now names it, says
where the count comes from (`sourceEventSeqs`), and why it belongs with
progress rather than here.
Adds the prerequisite: the terminal's replay and live replacement branches
are textually identical and 600 lines apart, so marker content needs one
home — fold them into a single `renderReplacement(event)` before giving the
row a payload that must stay consistent across both paths.
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