Byte-identical parallel block encoding in front of a sequential writer

If you move per-block compression in a streaming waveform writer onto worker threads, you can keep the output bytes identical to the single-threaded writer. Three rules do it. (1) The producer decides block boundaries using exactly the writer's rule. Here a full block closes only when time advances, so one timestamp's changes never straddle two blocks. (2) A single committer thread adds the encoded blocks in global job order, and cut/close markers ride in that same queue. Per-signal order and segment boundaries then come out the same. (3) Blocks force-closed by a cut or close must not trigger the size-based flush, because the sequential cut() closes every partial block and then writes one segment. When the block rule guarantees a block's last row is the settled value at its last time, the 'last kept value' needed for per-block change counts is just the previous block's last row, so workers don't need shared state. Verify with a fixed file id and cmp across block size, segment size, cut interval and worker-count grids.