Pure session layout and scrolling, shared by terminal and future graphical viewers.
Rows are indexed by logical UTF-8 offsets, not terminal-sized global scroll coordinates.
Character wrapping is explicit: these exact rows render without a second wrapping pass.
Grapheme boundaries keep CJK, combining characters and emoji intact. Control characters
are omitted just as ratatui’s text renderer omits them; an over-wide grapheme is skipped.