Render throughput as 'host → bytes @ rate │ bytes @ rate → host'

Two visual upgrades to the per-side throughput output, applied to both the live --progress ticker and the end-of-run --stats line:

• Replace the bare "source"/"target" labels with the actual URL hostnames, captured once during newConn via setSideDisplay. The internal label is preserved for ordering and arrow placement.

• Bracket each rate with a directional arrow and join the two halves with a vertical bar. Source flows left-to-right with the hostname on the left of its rate; target flows left-to-right with the hostname on the right of its rate. The result reads as a pipeline diagram: "github.com → ... │ ... → cloudflare.net".

Long hostnames (>30 chars) are truncated apex-preserving so the TLD stays visible — e.g. "8b04592ed74a5cce30d355b07276caf3.artifacts.
cloudflare.net" renders as "8b04592ed74a5cc…cloudflare.net". Falls back to plain right-truncation when the apex alone doesn't fit.