Track per-side transport bytes for source and target

Wraps both request and response bodies in the existing countingRoundTripper so byte counters update as data streams, not just at round-trip completion. Per-side counters keyed by transport label ("source", "target") feed an atomic byte total plus an elapsed nanos sample on each Stats snapshot.

Source totals are dominated by upload-pack response bytes (what we got from the source); target totals by receive-pack request bytes (what we pushed to the target). Counters are tracked unconditionally so a future live progress reader does not need ShowStats=true.