writeRowsViaWriter

private fn

string[] writeRowsViaWriter(scope  RenderedColumn[]  rendered,
       size_t rows,  size_t width,  CsvDialect dialect,  size_t segRows,
       size_t nSegs)

The writer fallback: the original segment loop streaming every row through ddn.data.csv.CsvWriter over a per-segment buffer. Retained for dialects the fast path does not take (currently only the invalid delimiter == quote dialect, whose INVALID_DIALECT failure message must stay the writer's own) and as the differential-test oracle.

Parameters

`rendered`	the column-major rendered columns
`rows`	the frame's row count
`width`	the frame's column count
`dialect`	the output dialect (LF newline already forced)
`segRows`	rows per segment
`nSegs`	the segment count

Returns

the per-segment output blocks, in row order.

Throws

DataFrameException with the row on any writer failure.