Erasing a wrapped wide character at the start of a row (ECH or DCH)
also clears the spacer head it left at the end of the previous row,
but that row was never marked dirty. With both rows visible, an
incremental render kept the stale spacer head on screen until
something unrelated redrew that row.
The clearing happens in the row-start branch of splitCellBoundary.
clearCells doesn't do dirty tracking, and both callers only mark
the cursor row, so mark the previous row at the point it's mutated.
The added dirty assertions fail without the fix.
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