csv_writer.nx

csv_writer.nx -- RFC 4180 CSV serialiser. Companion to csv.nx (reader). Writes cells with quoting that csv.nx can roundtrip: - Cells without any of \",\\n\\r,\" are emitted raw - Cells containing any of those are wrapped in \"...\" with embedded \" doubled to \"\" - Lines end with CRLF per RFC (not bare LF) Stream API: csvw_field(out, cap, off, data, n) -> off' (writes cell) csvw_end_row(out, cap, off) -> off' (writes CRLF) Caller tracks \"is this the first cell on the line\" by comparing the returned offset to the offset before writing; csvw_field inserts a \",\" separator when writing into the middle of a row. To simplify, we expose csvw_begin_row which resets the comma- separator flag stored on a CsvWriter state struct. Invariants: CW1 Roundtrip safe: csv.nx parsing our output yields the original cell bytes (after csv_decode_quoted for quoted cells). CW2 Empty string fields are OK -- they write as a bare \",\" boundary (or \"\"\"\" at end of row for unambiguous parsing per RFC 4180 §2.4). CW3 Output uses CRLF (0x0D 0x0A) line endings.