LOAD_STARTING_METHOD]: the size of the transcript a load is
about to replay, measured before any of it is deserialized.
The three size fields are None when the aggregate could not be computed
at all; [Self::intent_failed] then says why. A partially readable
transcript (a corrupt page under a handful of rows) still reports the
totals over the rows that could be read, with [Self::unreadable_nodes]
counting the rest.