batch.sx: persist/append-batch commits (type at data) specs as one contiguous
block; persist/append-batch-expect checks the stream is still at expected
before writing any event, so the batch is all-or-nothing under a concurrent
writer (conflict is a value, not a partial write). 162/162.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
