The around route reads forward from a prompt, so when a single turn holds
more display rows than the page limit, the previous prompt's page ends before
the open window's first row. Merging the two would paint one continuous
transcript with a silent hole. Each page now carries `pagination.offset`
(display rows before its first row); an older page that does not reach the
current offset replaces the display page instead of being prepended — the
reader still moves backwards, nothing is omitted without notice — and the
prepend scroll anchor is not spent for a replacement.
Docs: the desktop guide now says Show earlier keeps paging back after a
timeline jump.
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