Function format_rate 

pub fn format_rate(bytes_per_sec: f64) -> String

Formats a byte-per-second rate for display ("1.2 MB/s").

Delegates to crate::network::format_bytes so the unit vocabulary matches the Net field’s existing RX:/TX: totals rather than introducing a second scheme alongside it. Non-finite and negative inputs render as 0 B/s; they cannot arise from compute_rates, but the formatter is public and should not print NaN B/s.