Reserve a left gutter for absolute line numbers, drawn on each logical line's
first display row and blank on wrapped continuation rows. Gutter width tracks
the buffer's line count (2 digits + separator, widening past 99 lines) and
steals its columns from the soft-wrap. Widen the writing region 60->63 cols so
the gutter doesn't narrow the text: a file up to 99 lines keeps a full 60-col
text column, at the cost of the side panel shrinking 30->25 cols.
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