writeRowsFast

private fn

string[] writeRowsFast(DataFrame df,  size_t rows,
       string nullValue,  RowRenderSpec spec,  size_t segRows,  size_t nSegs)

Renders the rows of each segment straight into the segment's own buffer through the fast path: no CsvWriter, no FieldView, no intermediate RenderedColumn - every cell is formatted or copied exactly once, with the quoting rules of spec. Workers write disjoint blocks slots only, so the concatenation is byte-identical whatever the scheduling (ADR-2026-10-19-02).

Parameters

`df`	the frame to write
`rows`	the frame's row count
`nullValue`	the text of a null slot
`spec`	the dialect's rendering tables
`segRows`	rows per segment
`nSegs`	the segment count

Returns

the per-segment output blocks, in row order.