The prefetch producer thread fires a BytesRead event after every
batch it reads, carrying a cumulative byte count. Nothing asserted
this callback actually fired, so a consumer that passed None for the
event_fn would silently get no progress events. Add a fixture-based
regression test that drives a finite extent through new_with_events,
captures the events, and asserts the cumulative count is
non-decreasing and reaches the full extent size at EOF.
This locks the contract autorip's mux progress bar and soft-stall
watchdog depend on.
Matthew Jackson
f596dcb40e