DKVPX reader hard-codes comma as field separator, ignoring --ifs

Surfaced while investigating #369: the quote-aware DKVPX reader hard-codes its field separator to comma (`dkvpxReader.Comma = ','` in `pkg/input/record_reader_dkvpx.go`), so `--ifs` is silently ignored for this format. E.g. semicolon-separated quoted DKVP data read with `--ifs semicolon` is still split on commas, with no warning. Worth checking at the same time whether `--ips` is honored and whether any DKVPX writer path shares the problem. Expected: DKVPX should honor the resolved IFS (with a clear error for multi-character separators if the underlying reader requires a single character, mirroring the CSV reader's validation). A draft PR with the fix is in progress. 🤖 Generated with [Claude Code](https://claude.com/claude-code)