Pack blobs

Store the inline payloads of the spql.Blob columns in the Arrow rows of the body as blobs of a Table, packed as the rows of append_rows are, and return the rows with each payload replaced by its address, spiral://<table-id>/<key>, its offset, size and BLAKE3 digest. Payloads share packs in the Table's blobs/ directory, or in the attempt's; one of 4 MiB or more is an object of its own. Other values are returned as they are. The response starts once every pack is stored. The blobs become part of the Table only when a committed row references them. With an attempt, the binding the Tables API issued for that attempt is enough.