Content prefix before the change, used to highlight DIFF messages.
Bounded by diff.MAX_HIGHLIGHT_CHARS as trimmed by
highlight_source_prefixes on the way to the widget; from_widget reads the
already-trimmed value.
__post_init__ enforces the same length limit, but not the same rule: it
slices characters, so a value it truncates can end mid-line, which
highlight_source_prefixes — keyed on the diff's line numbers — never
produces. Such a prefix lexes into a partial final line and trips the drift
check in _highlighted_rows. It is a backstop against a direct constructor
call parking an unbounded copy of a file in a store whose whole point is
that thousands of messages cost little, not a second way to build a prefix.
