Function v2_to_legacy 

pub fn v2_to_legacy(
    batch: &RecordBatch,
    target: Magic,
) -> Result<Bytes, LegacyRecordsError>

Down-convert a v2 [RecordBatch] to v0/v1 MessageSet bytes.

If the v2 batch carried gzip or snappy compression, the output uses that codec in a wrapped MessageSet. The output keeps LZ4 in the same way. v0/v1 cannot represent Zstd, so this function emits an uncompressed MessageSet for a Zstd batch. The v2 decode path has already decompressed those records.

Errors

Returns an error if an absolute offset or timestamp exceeds i64, or if this function cannot encode the legacy message set.