Show per-side throughput at end of run and via --progress ticker

Two new outputs surfacing the per-side counters from the prior commit:

• statsLines now appends a 'throughput: source=X @ Y/s · target=A @ B/s' line whenever --stats produced non-zero per-side bytes. Computed against the stats collector's own wall-clock window so it works without --measure-memory.

• A new --progress flag wires a goroutine-driven stderr renderer that refreshes the same per-side line every 200ms in place. Auto-suppressed when stderr isn't a TTY so JSON pipes and CI logs stay clean. The reporter is started in newSession and torn down via a deferred syncSession.finish() in each public entry point so the final frame reflects the closing byte counts before the newline is emitted.

Plumbed through unstable.AdvancedOptions.Progress and exposed on sync, replicate, plan, bootstrap, and fetch.